Abstract: Ammonium polyphosphate(APP) was modified by aminosilane coupling agents. Modified ammonium polyphosphate was characterized by water solubility test, X-ray photo-electron spectroscopy(XPS) and thermogravimetric analysis(TGA). The results showed that modified ammonium polyphosphate has good hydrophobicity; a new bond was formed between aminosilane coupling agents and APP; the rate of thermal weight loss of modified ammonium polyphosphate decreased significantly. Limiting oxygen index(LOI) and mechanical property tests showed that the flame retardant and mechanical properties of the intumescent flame retardant PP with modified ammonium polyphosphate and dipentaerythritol(DPER) were improved apparently. The method of surface modificated APP by aminosilane coupling agents is not only simple and environmental friendly, but also decreases water solubility of APP and impr-oves the flame retardancy and interface compatibility of the intumescent flame-retardant PP.