After reading this post, it can be applied to treegrid as well, using queryParams. It's solved.
http://www.jeasyui.com/forum/index.php?topic=2343.msg5239#msg5239i'm still curious why it caused error. The doc says :
Reload treegrid data. If the 'id' parameter is passed, reload the specified tree row, otherwise reload all tree rows.
Code example:
$('#tt').treegrid('reload', 2); // reload the row which value is equals to 2
$('#tt').treegrid('reload'); // reload the all rows
$('#tt').treegrid('reload', {id:2, q:'abc'}); // reload the specified row with 'q' parameter passing to server