Abstract: In order to explore the causes and influencing factors of defects in the manufacturing process of casting products for motor train unit, macroscopic observation, metallographic examination, mechanical property test, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, fracture analysis, trace analysis and other methods were used to analyze the defects of casting products for multiple motor train unit. The results show that the defects caused by casting molding process, heat treatment process and repair process in the manufacturing period of casting products mainly included original casting defects, poor microstructure, original defects not eliminated after welding repair, and defects caused by poor machining.