With a motorcycle engine shell as the research object, analyzed the noise radiation of the engine shell by means of FEM / BEM, and determined the frequency range of the noise radiation and the critical region. Combining with sound intensity test, verified the correctness of the calculation result. The engine shell structure was optimized according to the numerical simulation results, and the noise radiation of the optimized model was predicted. Comparing the noise test results before and after the optimization, it is obtained that the surface radiated noise of the optimized engine shell is reduced about 4dB.