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As an important power machine in agricultural machinery, high-powered tractors have a wide range of speeds and large horsepower, which can be loaded with different agricultural implements to achieve diversified operations. During the field operation, the gearbox housing is subjected to various loads, including the coupling of stress amplitude and load frequency, and it is necessary to perform a fatigue analysis of the gearbox housing close to the actual situation. The concept of power density was introduced, and the fatigue life prediction method combining power density and time-frequency analysis was proposed. The effects of stress amplitude and load frequency on the fatigue life analysis of key components of high horsepower were studied. Taking 88kW tractor as research object, a dynamic strain test system was set up, and the load-time history of 88kW tractor under different operating conditions was collected. Based on the measured load, the fatigue life analysis method of the 88kW tractor gearbox shell was predicted by the fatigue life analysis method combining power density and time-frequency analysis. The fatigue life of the dangerous point was 24001h, compared with the fatigue life (35676h) obtained from the nominal stress method based on Miner theory, the actual working life was more accepted. The research result provided a more realistic analysis method for the fatigue life of key components of agricultural machinery and equipment, and promoted the development of fatigue analysis theory.