Salt gypsum is a waste residue produced in the salt industry.The main component of salt-gypsum is anhydrous cal-cium sulfate.It is necessary to add active agent to stimulate the activity of salt gypsum for its poor activity,slow setting and hardening rate,and low strength.The salt gypsum is modified by composite admixtures and its action mechanism is analyzed by XRD and SEM.The results showed that the single-doped flue gas desulfurization（FGD） gypsum or plaster of Paris could both shorten the setting time and increase the strength of salt gypsum,and the effect of plaster of Paris was better,but the dosage should not be too high.The addition of retarder could retard the setting time and decrease the strength of plaster,thus the amount of retarder should be appropriate as well.When the plaster of Paris and retarder were mixed,the ratio of them should be properly controlled and the suitable dosages of plaster of Paris and retarder were 50%（mass fraction,same as blow） and 0.10%~0.12%,respectively.The analysis showed that the high content of plaster of Paris will increase the amount of the dihydrate phase and the needle and bar crystals in the hardened salt gypsum;the increase of the retarder content will reduce the amount of the dihydrate phase in the hardened body of the salt gypsum,and the prismatic crystals will increase as well,thus reducing the strength of salt gypsum.