The work aims to verify the rigidity and strength of a certain D406A high-strength steel engine case for the combined instability condition of internal pressure and external load, and to ensure that the components meet the service strength requirements. The finite element software was used to establish an engine case stiffness finite element model, and numerical simulation analyses were carried out for different mesh densities and different structural cell types in the model to determine the parameters of the flexural deformation model with the best calculation accuracy. Under the combined working conditions of axial load and external pressure distributed load, the bending analysis and structural strength check were carried out, and the simulation showed that the single-layer solid mesh delineation had the best calculation accuracy, and the maximum stress value of the linear bending analysis was 67.2 MPa, the eigenvalue was 2.322, and the coefficient of structural residual strength was 1.39. In conclusion, the D406A high-strength steel engine case has sufficient strength, stiffness and stability under the service environment. It meets the design requirements of the engine case packaging structure.