Could you reply to this email as soon as possible?

as 在这里是连词，表示"像...一样"的意思。 soon 是副词，意思是"很快、不久"。 possible 是形容词，意思是"可能的"。 as soon as possible 是一个固定短语，字面意思是"像可能的那样快"，即"尽快"的意思。