Phosphorus pentoxide, propylene oxide, etc. were used as raw materials to synthesize oligomeric hydroxypropyl ethyl phosphate flame retardant. The effects of dropping temperature and dosage of propylene oxide, reaction temperature, reaction time, reaction pressure on the acid value of the target product were studied. The results showed that the optimum conditions were as follows: mass ratio of propylene oxide to intermediate Ⅱ was 0.67:1, propylene oxide dropping temperature 15 ℃, reaction pressure 0.5 MPa, reaction temperature 70 ℃, reaction time 1.5 h, the acid Value of target product reached 0.6 mgKOH/g . Its structure and performance were characterized by FTIR, GPC, TGA and chemical titration. Subsequently, the synthesized flame retardant was then used to modify polyurethane rigid foam. TGA data showed the composite containing 10% product Ⅲ, the residual mass at 900 ℃ increased from 7.9% to 20.71%; its flame-retardant performance test data showed the composite containing 30% product Ⅲ achieved UL-94 V-0 rating with a LOI value of 27.1% and retained the basic mechanical properties of RPUF