The high-precision 3D thermal-structure simulation model of a SRM(Solid Rocket Motor)nozzle divergent cone was developed by using FEM(finite element method)method.The strain and displacement distribution of the exit structure under the complex condition of high temperature, high pressure and the actuator loading together were calculated and compared with the full scale nozzle ground firing test results.It indicates that the simulation results are in good agreement with the test results.It verifies the accuracy and validity of 3D simulation model efficiently.The maximum strain error of key bearing part of the exit structure is less than 15%.It can be used as the strength verification model of exit cone in engineering application.Baesd on this, the First-order optimization method was adopted to lighten the weight of the structure with numbers of ribs in hoop/axial direction as variable parameters.It was indicated by application that under the premise of satisfying the strength and stiffness, the exit cone after optimization achieved the designing target of 30.8% weight reduction.The conclusions above in this paper can provide some important guidance for nozzle exit structure design and safety tolerance prediction of solid rocket engines.