Abstract: Through the diagnosis analysis and improvement of the casting process of a silicon-molybdenum ductile iron exhaust manifold（the exhaust pipe）, the waste of casting shrinkage and porosity was reduced from 40% to 2%, and the technological yield of casting was also increased by 5%. The author described the process of solving the shrinkage and porosity of casting. It is proved that riser size, riser position, riser neck parameters and chemical composition have a very important impact on shrinkage and porosity defects of castings. The correct design of riser parameters is an important link to improve the density of SIMO ductile iron exhaust pipe castings, and it is also an important factor affecting the casting process yield.