The retarding effect of retardants in building gypsum depend on many factors such as pH value.Fineness of gypsum particles,gypsum type,water/gypsum ratio and hydration temperature.In this paper, the influence of pH value, fineness and sort of gypsum on the effect of retarders are studied. Results show that for the gypsum with different pH value, fineness and of different sort, the effects of retarders on its setting time and strength are different.