This paper simulates air flow in inlet manifold in some engine with CFD calculation method. Results coincide well with experimental data, which indicates that turbulence in bend is the principal factor of flow loss. On the basis of spatial arrangement, the structure of inlet manifold is redesigned and optimized. Cross section of bend is designed as oblate shape and the inlet cross-section of manifold is increased while its radius is increased at the same time. Flow resistance and flow rate after optimization are superior to the prototype machine.