Abstract: The polymethacrylimide (PMI) foam cored sandwich structure was prepared by using PMI foam, carbon fiber/epoxy resin(EW220/5258)composites panel. The absorption curves of PMI foam cored sandwich structure, EW220/5258 composite panel and PMI foam were investigated, and the effect of the hydrothermal condition on the compressive and dielectric properties of the PMI foam cored sandwich structure was also determined. The results indicate that the saturated hygroscopic time of the PMI foam cored sandwich structure is 96 hours and its absorption ratio is 1.7%, the EW220/5258 composite panel can protect its PMI foam from water to improve its hydrothermal resistance. Due to the plasticization of water, the compressive strength of the PMI foam cored sandwich structure decreases in hydrothermal environment with the retention ratio of 65.87%. Additionally, the initial stage has more influences on the properties than other stages of hydrothermal treatment. After hydrothermal treatment, the loss tangent of the PMI foam cored sandwich structure increases from 0.008 to 0.012 within the range of 7~18 GHz, while the dielectric constant is almost unchanged.