不是介词.
　　as if 还可用于省略句中 如果as if 引导的从句是“主语＋系动词”结构,可省略主语和系动词,这样as if 后就只剩下名词、不定式、形容词（短语）、介词短语或分词.
　　as if 从句的作用 1． 在look,seem 等系动词后引导表语从句.如：She looks as if she were ten years younger． 她看起来好像年轻了十岁.It seems as if our team is going to win． 看来我们队要胜了.2． 引导方式状语从句.如：She loves the boy as if she were his mother． 她爱这男孩,就好像她是他的母亲一样.