第一种方式：

.js 定义函数

  <script type="text/javascript">

        //格式化单元格提示信息

        function formatCellTooltip(value){

            return "<span title='" + value + "'>" + value + "</span>";

        }

  </script>  

、调用函数

 <table class="easyui-datagrid" style="width:400px;height:250px"
    data-options="url:'datagrid_data.json',fitColumns:true,singleSelect:true">
    <thead>
    　　<tr>
　　　　　　<th data-options="field:'itemid',width:80,halign:'center',formatter:formatCellTooltip">Item ID</th>
    　　</tr>
    </thead>
 </table>

源码：jquery-easyui-1.3.6.zip    例子是：jquery-easyui-1.3.6\demo\datagrid\formatter.html

 第二种方式：
1.html
<table id="dg">
</table>

2.js

$('#dg').datagrid({

            fitColumns: true,

            nowrap: true,

            striped: true,

            rownumbers: true,

            pagination: true,

            singleSelect: true,

            columns: [[

            { field: "itemid", title: 'Item ID', width:,halign:'center', formatter: function (value) {

                return "<span title='" + value + "'>" + value + "</span>";

            }

            }]]

       });

源码：jquery-easyui-1.3.6.zip 例子是：jquery-easyui-1.3.6\demo\datagrid\formatter2.html

js 文件 Jquery.easyui.tooltip.js：

(function ($) {

    function init(target) {

        var opt = $.data(target, "tips").options;

        var tips = $(".easyui-tips-hover");

        if (tips.length == ) {

            tips = $("<div/>").css({

                "position": "absolute",

                "border-radius": "5px",

                "-webkit-border-radius": "5px",

                "-moz-border-radius": "5px",

                "padding": "5px",

                "background": "#fff",

                "display": "none",

                "border": "1px solid gray"

            }).hide().addClass("easyui-tips-hover").addClass(opt.cls);

        }

        opt.content = (opt.content || $(target).attr("tooltip"));

        tips.appendTo("body");

        $(target).css("color", opt.wrapColor);

        $(target).hover(function () {

            tips.html(opt.content);

            var offset = $(target).offset();

            //var outerWidth = tips.outerWidth();

            //            if (outerWidth > 200) {

            //                tips.width(200);

            //            }

            var scrollTop = $(document).scrollTop();

            var tipsHeight = tips.outerHeight();

            var outerWidth = tips.outerWidth();

            var targetHeight = $(target).outerHeight();

            var top = offset.top - tipsHeight;

            var left = offset.left;

            if ((offset.top - scrollTop) < top || top < ) {

                top = offset.top + targetHeight;

            }

            var bodyClienWidth = $("body")[].clientWidth;

            if ((bodyClienWidth - left) < outerWidth) {

                left = bodyClienWidth - outerWidth;

            }

            tips.css({ top: top, left: left }).show();

        }, function () {

            tips.hide().width("auto");

        });

    }

    $.fn.tips = function (options, params) {

        if (typeof options === 'string') {

            return $(this).tips.methods[options].call(this, params);

        }

        options = options || {};

        return this.each(function () {

            var opt = $.data(this, "tips");

            if (opt) {

                $.extend(opt.options, options);

            } else {

                $.data(this, "tips", {

                    options: $.extend({}, $.fn.tips.defaults, options)

                });

                init(this);

            }

        });

    };

    $.fn.tips.defaults = {

        cls: "",

        content: null,

        wrapColor: "blue"

    };

    if ($.parser) {

        $.parser.plugins.push('tips')

    }

})(jQuery);

<table id="dg"></table>

$(function () {

            var _grid = $('#dg').datagrid({

                url: 'products.json',

                columns: [[

            { field: 'productid', title: 'productid', width:  },

            { field: 'productname', title: 'productname', width:  },

            { field: 'price', title: 'Price', width: , align: 'right' }

            ]],

                onLoadSuccess: function () {

                    var tableTd = $('div.datagrid-body td[field="productname"]'); //productname是列名

                    tableTd.each(function () {

                        var $this = $(this);

                        var index = $this.parent('tr').attr('datagrid-row-index');

                        var rows = _grid.datagrid('getRows');

                        var currentRow = rows[index];

                        var content = '<div style=" max-width:700px;word-break: break-all; word-wrap: break-word;">' + currentRow.productname + '</div>'; //productname是列名

                        $this.tips({ content: content, wrapColor: 'black' });

                    });

                }

            });

        });

源码：jquery-easyui-1.3.6.zip    例子是：jquery-easyui-1.3.6\demo\datagrid\tips.html