Abstract: Phenyl polysiloxane microencapsulated ammonium polyphosphate (MAPP) was prepared by a sol-gel method using phenyltrimethoxy silane as a precursor. MAPP was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and thermogravimetric analyzer (TGA). The results showed that ammonium polyphosphate (APP) was successfully encapsulated with phenyl polysiloxane. The thermal stability and hydrophobicity of MAPP were significantly better than that of APP. MAPP and pentaerythritol (PER) were used together to improve the flame retardancy of polypropylene (PP). The results of limiting oxygen index (LOI) test demonstrated that the flame retardant property of PP/MAPP/PER composites were better than that of PP/APP/PER composites at the same loading. And the PP/MAPP/PER composites reached the UL-94 V-0 level. Moreover, the addition of flame retardant and carbon forming agent could promote the crystallization behavior of PP.