这个句子确实有错误.句子可能是 ：He was reading a book when it was raining.
那么提问的方式是：What was he doing when it was raining?
如果原句是：He read a book when it's raining.
那么提问的方式是：What did he do when it's raining?当然,这里的it's 是it was的省略方式