Four kinds of protein with similar molecular structure were chosen as the retarder of calcined gypsum from flue gas desulfurization(FGD) and phosphogypsum(PG) respectively and the retarding results were compared. The results show that the retarding degree of protein retarders with different amide groups is influenced by the type of calcined gypsum and increased with the increasing pH value of calcined gypsum. When the secondary amide is the main amide group of protein, the retarding degree is reducer as the amount of H2PO-4 increaser. When the primary amide is the main amide group of protein, the retarding degree is not influenced by H2PO-4. So retarder with the secondary amide as the main amide group of protein is suitable for calcined gypsum from FGD, and retarder with the primary amide as the main amide group of protein is suitable for calcined gypsum from PG.