In order to estimate the fatigue life of structures under variable amplitude loading, based on the dynamic residual S-N curve, the cumulative fatigue damage of materials under loading was quantified combined with the degradation law of materials, and a linear fatigue damage prediction model under variable amplitude loading was proposed. According to the fatigue life test data of hot-rolled 16Mn steel and 20Cr2Ni4A standard cylindrical gear under multistage load, the fatigue life prediction accuracy of the proposed model was verified. The results show that the prediction results of the linear fatigue life prediction model based on strength degradation is closer to the experimental results than those of the traditional Miner’s rule and material memory degradation accumulation model, showing higher prediction accuracy.