Abstract: In this paper, ammonium polyphosphate (APP) was surface modified with octadecylamine and the wettability was measured by static contact angle. The contact angle reached 136°, which indicated the modified APP possessed excellent hydrophobic property. APP and modified APP mixed respectively with char-foaming agent (CFA) with the ratio of 4:1 and added to polyethylene (PE) to prepare flame retardant PE materials. The flame retardant property is tested by the limiting oxygen index (LOI) and UL-94 test, the mechanical property is measured by tensile and bending, and the water resistance of the materials is tested by the boiled method. The results showed that, compared with the unmodified APP, the surface modification of APP slightly decreased the flame retardancy of the PE materials, but improved the compatibility of the flame retardant and polymer which resulted in the enhance of the mechanical property of the flame retardant PE materials. Moreover, the surface modification of APP greatly enhanced the water resistance of the flame retardant PE, and the extraction rate of the flame retardant material is only 0.12% when the loading of the flame retardant is 25%.