加上参数：nowrap: false,即可。 如下代码：

$("#userList").datagrid({
width: 'auto',
height: 'auto',
fitColumns:true,
nowrap: false,
collapsible: true,
singleSelect: true,
rownumbers: true,
url:"test/overload!getList.action",
columns: [[
{"field":"id","title":"","width":120,"align":"center", "hidden":true},
{"field":"name","title":"姓名","width":300,"align":"center"},
{"field":"age","title":"年龄","width":100,"align":"center"},
{"field":"unit","title":"单位","width":300,"align":"center"}
]]
});