The effect of compound retardant powder P on the hydration process,ion concentration in liuquid and supersaturation degree,and crystal morphology of dehydrate of FGD building gypsum is investigated.Furthormore,by using the SEM,its retarding mechanism is also studied.The results indicate that the addition of powder P retardant the early hydration of FGD building gypsum,which leads to delay of hydration heat evolution.It can also decrease the hydration ratio at early stage and prolong induction period of FGD building gypsum.Powder P has a major effect on the crystal morphology of dehydrate,moreover it enlarges crystal sizes.Retardation effect of powder P is worked by a variety of ways. The ionization of calcium ion from the hydroxyl carboxylic acid calcium salt definitely impedes the early dissolution of hemihydrate FGD building gypsum, delay the formation of nucleus.Also the selective absorption of groups composed of carboxyl and hydroxyl inhibits the growth of nucleus.Moreover the protein also plays a colloid coated role for gypsum,inhibiting the growth of nucleus.