Abstract: Early fracture failure happened to a leaf spring produced by a factory during the bench fatigue test. The fracture causes of the leaf spring were analyzed by macroscopic analysis, scanning electron microscopy and energy spectrum analysis, and metallographic examination. The results show that the fracture of the leaf spring was early fatigue fracture, and the fracture originated from the slag defect in the steel plate surface. The slags are mainly foreign inclusions, and they are mostly from the secondary oxide of molten steel when contacting with the ladles, tundishes and nozzles, including the deoxidation products which were not fully floated. By adopting measures such as converting low carbon catching to high carbon catching during converter smelting, using deoxidizing agents with a high carbon content such as lime during refining, and using a strong deoxidizer such as aluminum or silicon in the converter, the early fracture failure of the leaf spring due to the slag defect in the spring flat steel was effectively avoided.