Abstract: The radiation effect of different conditions on polymethyl-vinyl siloxane foam (PMVSF) under the state of pressure was investigated. The samples were firstly irradiated by ⁶⁰Co γ-ray, and then were degradated in the moist-heat environment and in the thermal oxidative environment, respectively. The correlation of PMVSF’s microstructure and macroscopic properties were determined by scanning electron microscope, Fourier transition infrared resonance, solid nuclear magnetic resonance (S-NMR), universal material tester and electron creep/relaxation tester, respectively. There is a dropping trend of the pores in irradiated samples. The result of ²⁹Si cross polarization SNMR shows that the peak intensity of organic silicon chain increases along with the increment of absorbed dose. The result of ¹³C cross polarization SNMR shows that the structure regularity of samples drops. It has been found that the tensile strength of irradiated samples after thermal degradation is lower than that of irradiated samples after moist-heat degradation. The synergy effect of radiation degradation and the moist-heat degradation is prominent to the samples’ stress relaxation ratio.