Abstract: Fiber reinforced resin-based composite stiffened panels were treated in a hygrothermal environment until they reached a moisture adsorption balance at 70 ℃ in a water bath. Then the effect of hygrothermal environment on shearing property of composite stiffened panels was studied in shear experiments for the stiffened panels before and after moisture adsorption. Results show that there is no obvious buckling in the test for two kinds of stiffened panels. The failure modes mainly contain damage of four corners, cleaving of skin and deformation of specimen and so on. The effect of hygrothermal environment on shearing failure load was so remarkable that it decreased by 9.9% after the composite stiffened panels absorbed moisture.