Abstract: The 38CrMoAl steel barrel used for injection molding machine had a large number of black spots on its surface after the machining process, Use methods such as macroscopic observation, scanning electron microscope analysis, and metallographic inspection to analyze the causes of black spots on the surface of the barrel. The results showed that the surface of 38CrMoAl steel barrel for injection molding machine was not uniformly nitrided. During machining process, intergranular cracking and peeling of the nitride layer occurred in the thicker area of nitride layer on the surface of barrel. This was the main reason for the black spots on the surface of the barrel.