the effects of three types of retarders such as citric acid, sodium polyphosphate, as well as bone glue on crystal morphology and pore structure of hardened gypsum paste have been systemically studied and then the internal cause of strength loss are put forward. The results show that the addition of retarders reduces supersaturation degree in liquid phase during early hydration of hemihydrates, and causes the change of the growth habits and transformation of crystal morphology of dihydrate gypsum from need-like to short prismatic, which correspondingly leads to large size crystals and weakening of the interlacing together of individual crystals. As a result, pore structure of hardened gypsum paste is worsened, which is mainly embodied in the increase of pore size and macro-pore (＞1μm ) fraction to a large extent, ultimately leading to the strength loss of hardened gypsum paste.