The wide application of composite materials in aerospace engineering makes it critical to evaluate the strength of the composite structures, especially for the simulation of thick laminate joint failure. Based on 3D continuum damage mechanics approach, this paper presents a study for modeling the damage mechanisms in a thick composite joint structure. Also, a user-defined FORTRAN subroutine (UMAT) is written to import the constitutive relation, failure criteria and stiffness degradation model. The substantiation test of the thick laminate joint has also been conducted. The calculation results show that the model could exactly simulate the progressive failure process subjected to tension load, which demonstrates the validity of this intra-laminar damage model for variable parameter iterative calculation and optimization design for thick composite joint structure.