A novel unsaturated phosphate ester (UPE) was synthesized with triethyl phosphate (TEP), ethylene glycol (EG), and alpha-methylacrylic acid (MAA). The unsaturated phosphate ester polymer (PUPE) was prepared by bulk polymerization of UPE. The structure of UPE and PUPE series were confirmed by Fourier transform infrared and liquid chromatography-mass spectrometry (LC-MS). Due to the chain transfer during polymerization, a crosslinked network was successfully introduced into the polymers, as evinced by the results from extraction and dynamic mechanical analysis. PUPE series exhibited high transparency, good mechanical properties, thermal properties, and excellent fire retardancy. The transmittance of PUPE-3 was up to 92%; the tensile strength and impact strength were 14.62 MPa and 30.5 kJ m(-2), respectively. The tensile strength of PUPE series was increased from 1.7 MPa to 35.5 MPa while impact strength was decreased with the increase of MAA ratio. The char yield of PUPE series reached 29.07 wt% by thermogravimetric analysis testing, which indicated that PUPE had excellent charring ability at high-temperature insulated air and prevented heat conduction. PUPE-3 had two glass transition temperatures (T-g) (113 degrees C/139 degrees C) as the crosslinking interaction restricts the polymer chain motion, and successfully passed UL-94 V-0 flammability rating and the LOI value reached 32%. POLYM. ENG. SCI., 59:E425-E431, 2019. (c) 2019 Society of Plastics Engineers