Please look at this example. It works fine.
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Load Form Data - jQuery EasyUI Demo</title>
<link rel="stylesheet" type="text/css" href="https://www.jeasyui.com/easyui/themes/default/easyui.css">
<link rel="stylesheet" type="text/css" href="https://www.jeasyui.com/easyui/themes/icon.css">
<script type="text/javascript" src="https://www.jeasyui.com/easyui/jquery.min.js"></script>
<script type="text/javascript" src="https://www.jeasyui.com/easyui/jquery.easyui.min.js"></script>
<script>
$(function () {
$('#fm').form('clear');
$('#fm').form('load', {
name: 'name1',
weekdaysList: ['2', '3']
})
})
</script>
</head>
<body>
<div id="dlg" class="easyui-dialog">
<form id="fm" class="easyui-form">
<input class="easyui-textbox" name="name" label="Name:" labelPosition="top">
<div id="weekdaysList" name="weekdaysList" class="easyui-checkgroup" data-options="
labelWidth: 100,
data:[
{value:'1',label:'Monday'},
{value:'2',label:'Tuesday'},
{value:'3',label:'Wednesday'},
{value:'4',label:'Thursday'},
{value:'5',label:'Friday'}
]"></div>
</form>
</div>
</body>
</html>