The stiffened composite panels still have enough strength after the bucking. Thus the study of the ultimate strength is very necessary. A shell-solid finite element model is built by Abaqus software to analyze the ultimate strength. The nonlinear finite element method is used to study the model at the compressive axial loading condition. In the model, fiber and matrix failure are considered, cohesive elements are built to simulate the debond failure between skin and stiffener. User subroutine is leaded into the analysis about the failure of materials. In the user subroutine, Quads failure criteria is adopted to identify the occurring of damage of the cohesive elements, the composite panels failure are identified by Hashin failure criteria, and the degradation criterion is used to simulate the failure progress of the stiffened composite panels. The progress of three failure modes are discussed detailed in the paper. After the simulation of the model, the debonding damage is proved as a important factor leading to the eventually failure.