When a tractor is working in the field, its key components are usually subjected to harsh geographical environment and weather conditions, moreover, the complex and variable loads, these make the key components of the tractor prone to fatigue failure. In order to accurately predict the fatigue life of large-horsepower tractors and provide theoretical basis for the design of key components of tractors, the stress field intensity method was adopted to analyze the fatigue life of steering drive axle housing of an 88kW tractor. The fatiguelife influencing factors, including stress concentration, size effect, surface quality and load characteristics were considered to optimize the traditional stress field intensity method. Traditional stress field intensity method was optimized from three aspects, i.e. the relative stress gradient, the fatigue damage area, and the measured load stress ratio, in order to obtain the modified S-N curve. The modified S-N curve was combined with the field measured load data to perform the fatigue life analysis of the steering drive axle housing, followed by comparing the prediction results of the optimizing stress field intensity method with the traditional method. The fatigue life of the dangerous point predicted by the optimizing method was 25467h，while the fatigue life obtained from the traditional method was 31860h. The comparison results showed that the optimized stress field intensity method was more accurate, since the perdition result was closer to the actual working life (24000h). The research result provided a more practical method for predicting the fatigue life of the key components of agricultural machinery equipment.