不对.
must表示必须时,
其反义疑问句用needn't,
表示推测时用isn't或aren't后加主语,
例如：He must be in the classroom, isn't he?
They must be at work, aren't they?
这两句不能用mustn't.
用May引导的请求许可的句子否定回答方能用mustn't,
例如：A: May I come in?
B: Yes, you May. （肯定回答）
B: No you mustn't! （否定回答）