This study focuses on the three-point bending property of the sandwich structure with resin-based carbon fiber composite face sheet and PMI foam core. The bending properties of three specifications specimens under room temperature and hygrothermal condition were compared. The difference between bending properties of specimens with different thickness ratio of face/core was investigated, and the influence of hygrothermal condition on the bending property of specimens was discussed. The results show that, under room temperature, the failure modes of core indentation， top face sheet fracture and top face sheet microbuckling were prone to appear that result in lower utilization rate of core, when specimen with relatively low thickness ratio of face/core. Meanwhile, when the thickness ratio of face/core is comparatively high, the failure modes of shear of face/core interface and plastic shear of core preferred to happen. On the other hand, too thick face sheet lead to excessive performance of face sheet. As for the specimens with hygrothermal treatment, due to the plasticizing effect of water, the compression strength and Young’s Modulus of PMI foam core became lower that set the failure of specimen under three-point bending load.