Abstract: The stiffened panels of CCF300 carbon fiber/BA9916-II epoxy resin composite were prepared. The compression behavior of the stiffened panel under dry and hygrothermal conditions was studied. The finite element model of the stiffened panel was established, and the material parameters in the hygrothermal environment were modified by the empirical formula. The compressive buckling and post-buckling behavior of the stiffened panel in different states were simulated and verified by experiments. The results show that the stiffened panel had strong post-buckling bearing capacity in both dry and hygrothermal states. The hygrothermal environment had a great negative effect on the stability and carrying capacity of the stiffened panel. With increasing moisture absorption time, the buckling and failure loads of the stiffened panel decreased fast and then slowly. The relative errors between the simulation and test resuls of buckling load and failure load of the stiffened panel in the dry state were 3.1% and 5.2%, and in the moisture saturation state were 5.6% and 6.9%, respectively. The errors were small, proving the accuracy of the simulation method and the reasonableness of the finite element model.