其实这里是伴随状语.一般伴随状语有两种情况：一种是ing形式,一种就是表语或被动语态.被动语态和表语也是可以做伴随状语的.其实该句子是两个合并的 He was sitting there ,and was lost in deep thought.短语be lost in 是陶醉于.的意思,是一个被动语态,也可以看成是表语.上面两句合并后,就变成了He was sitting there,lost in deep thought.