His expression is as earnest when he smiles as when he is arguing. 他不论笑的时候还是争论的时候表情都同样真诚。
【参考解析】这是as 从句的省略结构，在as…as结构中，当第二个as用作连词引导从句时，如果此从句是复合句,那么这个复合句的整个主句可以省略。所以你的句子完全句为：
His expression is as earnest when he smiles as it is when he is arguing. 他不论笑的时候还是争论的时候表情都同样真诚。
如果我们把 when 从句改为一个普通的时间状语，可能就更好理解一些：
I’m just as busy today as I was yesterday. 我今天和昨天一样忙。
【相似例句】
1. I told her I had never been in so many hotels in my life as since I came to Europe.
= I told her I had never been in so many hotels in my life as I had since I came to Europe.
我对她说我一生中从没象我到欧洲后住过张扬多的旅馆。
2. They had been talking for three hours without interruption and the President looked as fresh and alert as when they had started.(=as he looked when…) 他们连续谈了3个小时,总统看上去仍象谈话刚开始的时候一样,精力充沛,思维敏捷。
3. Bella came out of prison as haughty as when she went in. 贝拉从监狱出来，一如当初进去时一般傲慢。
4. Ibrahim left as bewildered as when he arrived. 跟他来时一样，易卜拉欣满腹狐疑地离开了。