Influence of one macromolecule-type retarder bone glue on the setting time, strength, ion concentration in liquid phase and supersaturation degree, crystal morphology of dehydrate and microstructure of hardened paste of building gypsum is investigated. Furthermore, internal cause of strength loss of building gypsum with addition of retarder is analyzed, The results indicate that bone glue could retard the hydration of building gypsum markedly, but its negative influence on the strength of building gypsum is far lower than that of low molecule-type retarders. Bone glue has a minor effect on crystal morphology of dehydrate and porosity of hardened paste, but it causes the large crystal size and coarsening of pore size distribution of hardened gypsum paste, which is the very intrinsic reason of strength loss of building gypsum.