Based on the analysis of boundary force of the pure-shear square plate in the post-buckling stage,the "I-shaped" composite beam test pieces considering the influence of the stiffness of skin supporting are designed to inspect the effect of post-buckling tension field on joint strength of composite beam by diagonal tensile test, and the finite element model based on cohesive zone model is used to simulate the experiment and study the interface failure process and mechanism. The results show that the delamination field generated in the post-buckling stage of the web significantly increases the delamination load at the interface, which leads to I/II complex stress state and accelerates the interface failure. The shearing test which considering the skin supported stiffness makes the skin form normal deformation after instability in the web, to some extent weakens the increase of the interface delamination load, so that the failure load of the beam slightly increases.The final failure mode is the failure of the web fiber compression and I/II complex fracture of the interface.The cohesive zone model can well simulate the fracture of the interface of the composite. And the simulation is basically consistent with the experiment.