According to the thermal structure coupling problem of composite nozzle of large diameter solid rocket motor, the subroutine of non-uniform load and non-uniform heat convection coefficient was written, and the 3D finite element method was used to obtain the distribution of temperature field and stress field in thermal protection material under the combined action of thermal load and internal pressure. The results showed that thermal load was the main cause of throat lining stress and shell stress. Secondly, small interfacial gap had no obvious impact on the internal stress intensity of thermal protective materials. Finally, the numerical method presented in this paper could provide guidance for the design of nozzle interface overlap and gap, and could also be applied to evaluate the thermal protection and structural strength safety margin of nozzle.