//判断是否是中文
        public bool IsChina(char c)
        {

            bool BoolValue = false;
            if (Convert.ToInt32(c) < Convert.ToInt32(Convert.ToChar(128)))
            {
                BoolValue = false;
            }
            else
            {
                return BoolValue = true;
            }
            return BoolValue;
        }
        //获取字符串中文数量
        public int GetLineIsChinese(Line list)
        {
            int isChinese = 0;
            for (int num = 0; num < list.Count;num++ )
            {
                if (IsChina(list[num].c))
                {
                    isChinese++;
                }
            }
            return isChinese;
        }
        //获取中文和英文差异宽度
        public int GetCharDiffWidth()
        {
            SizeF sizef = GetCharSize(this.Font, ‘中‘);//new Font("宋体", 10)
            int len = (int)sizef.Width - CharWidth;
            return len;
        }

   //draw selection
            if (!Selection.IsEmpty && lastChar >= firstChar)
            {
                e.Graphics.SmoothingMode = SmoothingMode.None;
                var textRange = new Range(this, from + firstChar, iLine, from + lastChar + 1, iLine);
                textRange = Selection.GetIntersectionWith(textRange);
                if (textRange != null && SelectionStyle != null)
                {
                    int chineseCount= GetLineIsChinese(line);
                    int diffWidths = GetCharDiffWidth()*chineseCount;
                    diffWidth = diffWidths;
                    SelectionStyle.Draw(e.Graphics, new Point(startX + (textRange.Start.iChar - from) * CharWidth, y),
                                        textRange);
                    diffWidth = 0;
                }
            }

public override void Draw(Graphics gr, Point position, Range range)
        {
            //draw background
            if (BackgroundBrush != null)
            {
             
                Rectangle rect = new Rectangle(position.X, position.Y, (range.End.iChar - range.Start.iChar) * range.tb.CharWidth+FastColoredTextBox.diffWidth, range.tb.CharHeight);
                if (rect.Width == 0)
                    return;
                gr.FillRectangle(BackgroundBrush, rect);
            }
        }

diffWidth  这是公共变量，放在FastColoredTextBox类的公共变量声明里面

public static int diffWidth = 0;

/*****************************/

/*  该代码用于准确定位字，主要是因为中文不能准确定位  */

/*****************************/

 /// <summary>
        /// Gets nearest line and char position from coordinates
        /// </summary>
        /// <param name="point">Point</param>
        /// <returns>Line and char position</returns>
        public Place PointToPlace(Point point)
        {
#if debug
            var sw = Stopwatch.StartNew();
#endif
            point.Offset(HorizontalScroll.Value, VerticalScroll.Value);
            point.Offset(-LeftIndent - Paddings.Left, 0);
            int iLine = YtoLineIndex(point.Y);
            int y = 0;

            for (; iLine < lines.Count; iLine++)
            {
                y = LineInfos[iLine].startY + LineInfos[iLine].WordWrapStringsCount * CharHeight;
                if (y > point.Y && LineInfos[iLine].VisibleState == VisibleState.Visible)
                    break;
            }
            if (iLine >= lines.Count)
                iLine = lines.Count - 1;
            if (LineInfos[iLine].VisibleState != VisibleState.Visible)
                iLine = FindPrevVisibleLine(iLine);
            //
            int iWordWrapLine = LineInfos[iLine].WordWrapStringsCount;
            do
            {
                iWordWrapLine--;
                y -= CharHeight;
            } while (y > point.Y);
            if (iWordWrapLine < 0) iWordWrapLine = 0;
            //
            int start = LineInfos[iLine].GetWordWrapStringStartPosition(iWordWrapLine);
            int finish = LineInfos[iLine].GetWordWrapStringFinishPosition(iWordWrapLine, lines[iLine]);
            var x = (int)Math.Round((float)point.X / CharWidth);
            x = x < 0 ? start : start + x;
            if (x > finish)
                x = finish + 1;
            if (x > lines[iLine].Count)
                x = lines[iLine].Count;

            if (lines[iLine].Count > 0)
            {
                x = GetXWidth(lines[iLine], point.X);
            }

#if debug
            Console.WriteLine("PointToPlace: " + sw.ElapsedMilliseconds);
#endif

            return new Place(x, iLine);
        }

        private Place PointToPlaceSimple(Point point)
        {
            point.Offset(HorizontalScroll.Value, VerticalScroll.Value);
            point.Offset(-LeftIndent - Paddings.Left, 0);
            int iLine = YtoLineIndex(point.Y);
            var x = (int)Math.Round((float)point.X / CharWidth);
            Line line = lines[iLine];
            if (x < 0) x = 0;

            if (line.Count > 0)
            {
                x = GetXWidth(lines[iLine], point.X);
            }
            return new Place(x, iLine);
        }
        //计算x轴应该显示第几个字的宽度
        public int GetXWidth(Line line,int x)
        {
            int iChar = 0;
            int len = 0;
            for (int num = 0; num < line.Count; num++)
            {
                if (len >= x)
                {
                    break;
                }
                if (IsChina(line[num].c))
                {
                    iChar += 2;
                    len += (GetCharDiffWidth() + CharWidth);
                }
                else
                {
                    iChar++;
                    len += CharWidth;
                }
            }
            return iChar;
        }

http://files.cnblogs.com/MyNameIsMT/FastColoredTextBox20140116.zip

修改FastColoredTextBox控件完成选择