Taking the composite structure of opposed piston opposed cylinder two-stroke diesel engine as the main study object, the effects of different bolt pre-tightening forces on structure rigidity were studied by using modal testing method and the effects of structure parameters and materials on the structure rigidity of crankcase and intake and exhaust pipes were studied by the finite element numerical simulation. The results show that the pre-tightening forces have little influence on the stiffness of composite structure. The increase of intake and exhaust pipe thickness has little contribution to improving structure rigidity. The material of intake and exhaust pipes that uses cast aluminum instead of cast iron can lighten the weight of composite structure and enhance the dynamic stiffness. The increase of crankcase rib and bulkhead thickness is beneficial to improve the structure rigidity, the material has little effect on natural frequency, and the aluminum alloy material should be preferred for the lightweight design.