Experiments for shear stability analysis were conducted on thick composite stiffened panels. The traditional engineering method was not conservative compared with the test results since the stringer was not rigid enough to isolate the buckling wave due to the thicker skin and relatively weak stringers. Finite element analysis showed that the buckling mode crossed through the stringer. A modified engineering method was proposed to predict the buckling load which smeared the stiffness of the stringer to the skin. The error of buckling loads calculated by modified method and nonlinear finite element method was − 8% and − 5% respectively. The nonlinear finite element analysis method was more accurate in calculating the buckling load compared with the linear buckling analysis. Interface delamination was easy to happen after shear buckling which would cause panel failure. The peeling force of the interface would increase significantly under the shear buckling resulting to adhesion failure. The post-buckling capacity of thick panel under shear load was low since the post buckling factor was 1.09 according to the test results, and then it was recommended to use the buckling load as design load.