In this study, the novel halogen-free flame retardants (PSiN, A and B), which contain phosphorus, nitrogen, and silicon, have been synthesized. The structure of PSiN-A is characterized by fourier transform infrared spectroscopy and proton nuclear magnetic resonance, and its thermal property is studied through thermo gravimetric analysis (TGA). PSiN-A and B were blended with polypropylene(PP) to obtain PP/PSiN composites. The flame-retardant properties of PP/PSiN composites are estimated by Limiting Oxygen Index (LOI) values, and their degradation behaviors are investigated through TGA, under nitrogen from room temperature to 800 degrees C. The fire performance of PP is improved by PSiN(A or 13): the LOI value of PP/PSiN-A reach 26.0 vol% and the char yield is at 27 wt% at 800 degrees C. The phosphorus in PSiN provides possibility for the PP blends to form char, and the silicon improves the thermal stability of char. The active energies of PSiN and PP are calculated through the method of Horowitz-Metzger.