Ext.onReady(function () {

    // Init the singleton.  Any tag-based quick tips will start working.

    Ext.tip.QuickTipManager.init();

    Ext.widget('grid', {

        title: 'Users',

        store: {

            fields: ['name', 'email', 'comment'],

            data: [

                { 'name': 'Lisa', 'email': 'lisa@simpsons.com', 'comment': 'some comment' },

                { 'name': 'Bart', 'email': 'bart@simpsons.com', 'comment': 'comment' },

                { 'name': 'Homer', 'email': 'home@simpsons.com', 'comment': 'some very long comment' },

                { 'name': 'Marge', 'email': 'marge@simpsons.com', 'comment': 'some very very very very long comment' }

            ]

        },

        columns: [

            { header: 'Name', dataIndex: 'name', width: 100 },

            { header: 'Email', dataIndex: 'email', width: 150 },

            {

                header: 'comment',

                dataIndex: 'comment',

                flex: 1,

                renderer: function (value, meta, record) {

                    var max = 15;

                    meta.tdAttr = 'data-qtip="' + value + '"';

                    return value.length < max ? value : value.substring(0, max - 3) + '...';

                }

            }

        ],

        width: 400,

        renderTo: 'output'

    });

});

参考的原文：http://ext4all.com/post/extjs-4-how-to-add-grid-cell-tooltip