Define the tooltip content on the tab title and then instance it after the tabs are created successfully.
<div class="easyui-tabs" style="width:700px;height:250px">
<div title="<span class='tip' title='tip content'>tab title1</span>">
</div>
<div title="<span class='tip' title='tip content'>tab title2</span>">
</div>
</div>
<script type="text/javascript">
$(function(){
$('.tip').tooltip()
})
</script>