I will reply to your email as soon as possible.

as 在这里是连词，表示"当...的时候"或"随着"。 soon 是副词，意思是"不久、很快"。 第二个 as 也是连词，与第一个 as 构成固定搭配。 possible 是形容词，意思是"可能的"。 as soon as possible 是固定短语，字面意思是"尽可能快地"，通常简写为ASAP，表示"尽快"。