In order to study the strengthening mechanism of stitching thread on the pull-out load-carrying capacity of stitched composite T-joints， a finite element model of unstitched composite T-joints is established. The numerical results are in good agreement with the existing experimental data. At the same time， according to the translational symmetry of stitched composite T-joints in the width direction， a simplified model of stitched T-joints is established by adding specific boundary conditions to the model boundary to simulate the progressive damage process of the structure. On this basis， the effects of the position and cross-sectional area of the stitching thread on the mechanical properties of T-joints under pull-out load are further studied. The results show that the closer the stitching thread is to the triangle area， the more influence of the stitching thread has on the maximum bearing capacity of the structure. When the stitching position of the stiffener and skin is at the edge of the triangle area， the stitching thread with the smaller cross-sectional area will weaken the bearing capacity of the structure， while the stitching thread with the larger cross-sectional area will greatly enhance it.