In order to research the fracture of cone root of steering knuckle arm occurring at the phase of durability test on the enhancement road of proving ground, and consider the characteristics of material structure and process assembly precision, the fracture model of steering arm is analyzed in detail. The full bridge strain gauge is pasted in the vicinity of fracture and the testing sensor linked to the steering system is equipped, thus the bending strain of the broken portion, lateral acceleration and displacement of steering tie rod and other load data are collected. The hysteresis loop curve equation of cyclic stress-strain and Neuber formula is applied to convert the nominal stress to the local stress-strain cycle response at the fracture location. According to Manson-Coffin equation, impact of mean stress on fatigue damage is corrected. The analysis and calculation results show that the material characteristic of steering arm meets the technical conditions, but the taper and surface finish of steering arm cannot meet the design precision in drawing, the contact area of steering arm and steering knuckle can only reach 30%, local high stress is caused at the steering knuckle and cone root of steering knuckle arm, and bending low cycle fatigue fracture will generate.