已经解决了这个BUG,得修改ext源代码,方法如下:
在ext-all.js中找到Ext.form.CheckboxGroup,
将 var panelCfg={cls:this.groupCls,layout:'column',border

alse,renderTo:ct};var colCfg={defaultType:this.defaultType,layout:'form',border

alse,defaults:{hideLabel:true,anchor:'100%'}}
var panelCfg={cls:this.groupCls,layout:'column',border

alse,renderTo:ct};var colCfg={defaultType:this.defaultType,layout:'form',border

alse,defaults:{hideLabel:true,anchor:'100%'}}
替换为
var panelCfg={cls:this.groupCls,baseCls:"x-plain",layout:'column',border

alse,renderTo:ct};var colCfg={defaultType:this.defaultType,layout:'form',baseCls:"x-plain",border:false,defaults:{hideLabel:true,anchor:'100%'}}
var panelCfg={cls:this.groupCls,baseCls:"x-plain",layout:'column',border:false,renderTo:ct};var colCfg={defaultType:this.defaultType,layout:'form',baseCls:"x-plain",border:false,defaults:{hideLabel:true,anchor:'100%'}}
既添加了baseCls:"x-plain" 属性