This paper deals with the effect of different waterproof agents for gypsum on the formation of gypsum crystal.Combining waterproof performance of gypsum with utilizing modern testing instrument,such as SEM,EPS,etc.,the reasons for influence of waterproof agents for gypsum on the crystal formation of gypsum products and its waterproof mechanism were analyzed in terms of crystal changing of gypsum after adding compound waterproof agent for gypsum.