We analyzed the influence of sodium polyphosphate (STPP) on the performances of recycled gypsum (RG). This analysis was performed with two different ways of STPP addition: One was that the STPP was added into Plaster of Paris (POP), then recycled plaster (R-SP) was obtained after a series of processes and the other was that the STPP was directly put into recycled plaster (R-PS). The conclusions confirmed that STPP increased the water demand and delayed the hydration of R-PS. With regard to its hardened performance, STPP provided hardened recycled gypsum with a loose structure which led to the lower strength and higher water absorption than recycled plaster (R-P) without STPP. On the contrary, the water requirement and the setting time were decreased and shortened by STPP in R-SP, respectively. A dense structure was also possessed by R-SP which contributed to the high strength and low water absorption. The analysis shows that STPP is selectively chemisorbed on the (111) face of R-PS crystals, it inhibits the growth in c axis direction, resulting in the extending of setting time and the transformation of morphology, making the hardened R-PS crystals coarsened, which results in high W/P and low strength. Whereas the low W/P is made by both the rod-like crystals of low length-radius possessed by R-SP and the relatively high ζ-potential absolute value caused by – (PO3Ca) on the surface of R-SP, leading to the complete development of crystals and their close overlapping, thus bringing about the increase of strength.