A novel halogen-free flame retardant aluminium hypophosphite (PAH) was synthesized by sodium hypophosphite monohydrate and aluminum sulfate octadecahydrate as base material． The optimal reaction conditions were obtained:The reaction temperature being 90 ℃, the molar ratio of sodium hypophosphite monohydrate to aluminium sulfate octadecahydrate being 7.5:1.0 and the reaction time being 3 h,the yield of aluminium hypophosphite was up to 98.5%.The structure of product was characterized by means of infrared spectrometer and inductively coupled plasma atomic emission spectrometer. The application of PAH to polyurethane foam was investigated.The results show that the flame retardant polyurethane foam has LOI 30 and UL94 V-0.This method was simple, environmental and easy to realize industrialization.