Abstract: A leaf spring of 65Mn spring steel fractured. In order to find out the failure cause of the leaf spring, macro and micro observation, microstructure examination, chemical composition analysis and microhardness testing were carried out, and the processing and operation of the leaf spring were analyzed. The results show that the fracture mode of the leaf spring is hydrogen-induced delayed brittle fracture. Damage occurred for lack of protective measures during the bending process, and a large number of hydrogen atoms accumulated at the bending position, which eventually led to the hydrogen-induced delayed brittle fracture. Perfecting the bending process to reduce stress concentration and appropriate reduction of hardness within the range of technical requirement can effectively prevent such failure, which has been verified.