The recycled gypsum sample with sodium polyphosphate (STPP) was prepared under the condition of laboratory simulation.Then the sample was calcined to hemihydrate and carried out the tests of its mechanical properties.By using modem testing instrument, such as SEM, Zeta potential analyzer, the effect of STPP on the performances of recycled gypsum and its change mechanism were analyzed.The results showed that the setting time and water-plaster weight ratio for standard consistency (W/P) of plaster of paris (POSP) with STPP were increased, which impaired the strength, while on the contrary, the setting time and W/P, strength of its recycled plaster (R-SP) were reduced and increased, respectively, compared with the recycled plaster (R-P) without STPP.And the strength was decreased to R-P and increased to R-SP, respectively, compared with POP and POSP.The analysis indicates STPP is selectively chemisorbed on the (111) face of gypsum crystal, it inhibits the growth in c axis direction, resulting in the transformation of morphology, thus making the hardened POSP crystals coarsened.When calcined, R-SP has rode-like crystals of low length-radius, also,-PO3Ca existing in the surface of R-SP increases the repulsive force between R-SP particles and improves their dispersion, which makes the W/P low, leading to the complete development of crystals and their close overlapping, thus reinforcing its hardened gypsum.