Abstract: The liquid oxygen tanks that made of carbon fiber/epoxy resin composites are vital for weight reduction in new generation spacecraft such as heavy rockets and space shuttles. However, the incompatibility of epoxy resin with liquid oxygen limits their application. Thermogravimetric analyser, the Kissinger method, the Coasts-Redfern method and thermogravimetric-infrared-gas chromatography/mass spectrometry were used to investigate the thermal degradation behaviors and mechanism of 10-(2, 5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphafi-10-oxide (ODOPB)-modified liquid oxygen-compatible epoxy resin (ODOPB-EP). The results show that the thermal degradation mechanism of ODOPB-EP is a phase boundary reaction, corresponding to the degradation mechanism function g(α)=1−(1−α)^1/3, α is conversion rate. During the thermal degradation process, the resin breaks from the weak bonds of C—N and C—O. With the increase of temperature, aromatic substances such as phenol and its derivatives are released. Besides, ODOPB part in resin produces biphenyl and other substances, accompanied by the release of phosphorus-containing radicals. The phosphorus-containing radicals could exert quenching effects, which is conducive to improving the compatibility of the resin with liquid oxygen. This study provides a theoretic basis for verifying the compatible mechanism of polymers with liquid oxygen.