Abstract: Carbon fiber reinforced vinylester resin composites (CFRP) are widely used as structural materials in ocean and ship engineering. In an unpredictable ocean environment and service condition, materials are subjected to hygroscopic environment and extreme temperatures. In this paper, the weight change and morphology changes of resin matrix and CFRP after immersion into water as well as the development of mechanical properties for CFRP during immersion duration at three temperatures (−30℃, room temperature and 70℃) were studied. The results from FTIR and liquid chromatography-mass spectrometry showed that the vinylester resin was hydrolyzed during immersion, and observations on the microscopic morphology revealed that the formation of fiber-matrix interface changed the moisture absorption behavior of resin matrix. The compressive strength of CFRP at cryogenic temperature and elevated temperature as well as at room temperature after 120 days’ immersion decreased by 27.4%, 36.2% and 32.8% as compared to the unaged strength at room temperature, respectively. And the in-plane shear strength increased by 35% at low temperature, decreased by 27% at elevated temperature, and increased by 7% after 120 days’ immersion, showing that the influence of temperature on the in-plane shear strength of CFRP was greater than that of hygroscopic aging. Meanwhile, the results from dynamic thermomechanical analysis displayed that the storage modulus and glass transition temperature (T_g) declined due to the moisture absorption, but later recovered partially.