Abstract: The absorption characteristics and the changes of the static mechanical properties and dynamic mechanical thermal properties of the pultruded carbon fiber/vinylester (CF/VE) composite, which is used for sucker rods in oil field, were studied during distilled water immersion at 95℃. The surface and fracture morphologies of the as-received and immersed composites were observed by using SEM. The results show that the composite obeys the second Fick's law in terms of absorption behavior, having an equilibrium moisture content of 1.046% and a diffusivity of 2.233×10^-6 mm²/s. The decrease in the properties of the composite is very relative to the increase in its moisture content. After being immersed for 1176 hours, the composite has a reservation of interlaminar shear strength (ILSS) and flexural strength of 54% and 49%. The results of dynamic mechanical thermal analysis (DMTA) and SEM show that hygrothermal environment causes the swell and plasticization of the matrix, interfacial debonding and resultant microstructure changes of the matrix and interface. No chemical aging was found.