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The present work involves the investigation of coupling effects of gamma irradiation and absorbed moisture on silicone foam. Compression stress-strain test, creep test, differential scanning calorimetry (DSC) and swelling test were performed to analyze the properties of pristine and dewatered samples irradiated by 60Co gamma ray. Changes in the compression stress, creep elongation, melting temperature, melting enthalpy and the average molecular weight between cross-links of the dewatered samples are much more obvious than that of the pristine ones. The possible mechanism was investigated by computational study with analog molecules. It suggests that the absorbed moisture and its radiolysis species (H·/HO·) would consume part of the macromolecular radicals generated by gamma irradiation, which hinders the crosslinking reactions. This should be the main mechanism for the coupling effects of moisture and irradiation at room temperature.