Actually i solved the problem. It was because the idField of the table i was using was incorrect.
it appears that the getSelections uses the idField property so you need to make sure that the idField is actually the same as the one passed in the json data.
每一行的 idField属性的值必须不一样，最好使用数据库主键
参考地址：
http://www.jeasyui.com/forum/index.php?topic=401.0
EasyUI datagrid.getSelections 没有返回正确的选择行数
原文：http://www.cnblogs.com/tangchun/p/7727409.html