Abstract: In order reasonably to estimate herringbone gear fatigue life through the dynamic stress of tooth root, a gear vibration model of 12-degree freedom of herringbone is established, considering meshing stiffness, corner mesh impact, and tooth surface contact friction. Meanwhile, the tooth-root-stress relevance between other meshing teeth is also considered, and the dynamic stress trend of herringbone gear tooth root of the tension side is calculated. The dynamic stress at the mid-point of the pinion tooth root of the tension side under three commonly used load conditions is selected as the fatigue life calculation object by analyzing the theory of tooth fatigue life mechanism. The dynamic stress fatigue life of herringbone gear teeth root is calculated by Miner linear cumulative damage theory and the counting method of two-parameter rain flow, considering stress amplitude and mean stress. The influence is discussed on the fatigue life of the tooth root reverse compressive stress between other meshing teeth. Herringbone gear tooth surface is optimized under tooth dynamic stress fatigue life to the consolidated load conditions by three-dimensional modification technology. The results show that the tooth root dynamic stress trend flattens after modification, and the fatigue life is increased by 25%.