using System;

using System.Collections.Generic;

using System.IO;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows;

using System.Windows.Controls;

using System.Windows.Data;

using System.Windows.Documents;

using System.Windows.Input;

using System.Windows.Media;

using System.Windows.Media.Imaging;

using System.Windows.Navigation;

using System.Windows.Shapes;

namespace textbox

{

    /// <summary>

    /// MainWindow.xaml 的交互逻辑

    /// </summary>

    public partial class MainWindow : Window

    {

        public MainWindow()

        {

            InitializeComponent();

            textarea.TextChanged += textarea_TextChanged;

            textarea.MaxWidth = 157;

        }

        private void textarea_TextChanged(object sender, TextChangedEventArgs e)

        {

            //textarea.TextChanged -= textarea_TextChanged;

            int pos = textarea.SelectionStart;

            string text = textarea.Text;

            string modifyTxt = getTextWithNewLine(text);

            if(text.Length != modifyTxt.Length)

            {

                int pos_x = lenb(modifyTxt) - lenb(text);

                textarea.Text = modifyTxt;

                if(pos%20 == pos/20)

                {

                    int val = pos + pos_x;

                    if(val < 0)

                    {

                        val = 0;

                    }

                textarea.SelectionStart = val;

                }

                else

                {

                    textarea.SelectionStart = pos;

                }

            }

                //Console.WriteLine(modifyTxt);

            //textarea.TextChanged += textarea_TextChanged;

        }

        int lenb(string c)

        {

            int lens = Encoding.GetEncoding("Shift_Jis").GetByteCount(c);

            return lens;

        }

        string getTextWithNewLine(string text)

        {

            char[] array = text.ToCharArray();

            StringBuilder sb = new StringBuilder();

            bool autoNewLine = true;

            int index = 0;

            int rowcont = 0;

            for(int i=0;i<array.Length;i++)

            {

                char c = array[i];

                if(c == '\r')

                {

                    autoNewLine = false;

                    continue;

                }

                else if (c == '\n')

                {

                    if (autoNewLine)

                    {

                        if(index < 19)

                        {

                            continue;

                        }

                    }else

                    {

                        sb.Append("\r\n");

                        index = 0;

                        rowcont++;

                        autoNewLine = true;

                    }

                }

                else

                {

                    if (index > 19)

                    {

                        sb.Append('\n');

                        index = 0;

                        rowcont++;

                    }

                    sb.Append(c);

                    index += lenb(c + "");

                }

            }

            return sb.ToString();

        }

    }

}

<TextBox x:Name="textarea" AcceptsReturn="True"  Width="157" Height="200" VerticalScrollBarVisibility="Visible" BorderThickness="1">12345678901234567890</TextBox>