Abstract: The specimen and corresponding cramping apparatus were designed for J shape composite stiffened plate under compression. The load-carrying capability and failure mode of the specimen were tested. Then the modeling and analysis methods for numerical investigation were discussed, and the buckling and post-buckling behaviors of the composite stiffened plate were studied by using finite element method (FEM). The prediction result of load-carrying capability of a structure, without consideration of the effect of cumulated damage on the capability, will be obviously higher than the experiment result. When considering failure criteria for the damage of matrix, fiber-matrix shear failure and fiber break during the post-buckling analysis, the accuracy of numerical prediction can be improved effectively.