Create a province combobox component and a city combobox component. Listen to the 'onChange' event on the province component and load new data for the city component. The code looks like this:
<input id="province" class="easyui-combobox" data-options="onChange:onProvinceChange>
<input id="city" class="easyui-combobox">
<script>
function onProvinceChange(value){
$('#city').combobox('loadData', ...);
// or $('#city').combobox('reload', ...);
}
</script>