At present, there is much research on the shrinkage defects of thick and large ductile iron castings. However, less research has focused on shrinkage defects in thin-walled parts, especially in complex structure castings. This paper analyses the cause of the shrinkage defects in the intake and exhaust pipes of a certain type of marine cylinder head and the slag inclusion problem on the spring seat surface of our company. Corresponding measures were taken to optimize the process: ① the isolated hot spots were eliminated by improving the cold iron process of the exhaust hole; ② the machining allowance was added on the spring seat surface so that the slag was fully floated and removed by machining. The subsequent machining results show that all defects in the cylinder head after the process optimization have been eliminated. The quality has been effectively improved, with the qualification rate increasing from 50% to 93%.