For the problems of easy torsion and unstable size of 59 mm×100 mm rectangular 52CrMoV4 spring flat steel, the design of hole pattern and the optimization of rolling process were conducted, and the failure cause of broken spring flat steel was analyzed by the methods of chemical composition analysis, microstructure analysis and mechanical performance test. The results show that the chemical composition of material meets the requirements. In the mechanical performance indicators, the tensile strength is 1511/1494 MPa, the yield strength is 1362/1344 MPa, the elongation after fracture is 11.5%/9.0%, the reduction of area is 37.0%/36.5%, the surface hardness is distributed between 42.4 and 42.9 HRC, the depth of surface decarburization layer is 0.24-0.28 mm, and the austenite grain size is 7.5-8.0 grades. No metallurgical defects such as large inclusions are found, and the main matrix structure of material after heat treatment is tempered martensite+a small amount of bainite. The fracture originates from the surface, and there are obvious indentations on the surface of fatigue source area, while no indentations are found in other positions. Therefore, the surface indentations are the direct cause for the fatigue life of material,and the microstructure of instantaneous fracture zone is intergranular+tearing dimples.