Abstract: To comprehensively illustrate the fatigue damage evolution mechanism, a pair of high-speed gears in a 1.5 MW wind turbine was selected as the research object. Based on the geometric similarity between Voronoi diagram and polycrystalline structure, a mesoscopic tooth surface model was established with Voronoi tessellation. Cohesive elements were inserted in the model to simulate the fragmented effect of grain boundary on matrix. The bilinear-separation curve being related to cohesive elements was used to evaluate whether the damage originates or not. Then, jumping-in-cycles (JIC) loading method was applied to simulate the cyclic cumulative effect of contact loading due to gear meshing. As a result, the initiation location and initiation life of contact fatigue crack were simulated and predicted, and the effects of wind speed change and friction coefficient on crack evolution were discussed. Results show that the damage evolution rate is relatively slow at the initial stage, and gradually increases with the loading process and damage accumulation. For the investigated gear and its working conditions, the first two cracks appear at the depth of 0.13 mm and 0.27 mm under the gear surface. Initial cracks grow gradually under cyclic loading of contact stress, and finally form typical surface spalling failure. Additionally, higher wind speed and worse lubrication condition will significantly reduce the life of wind turbine gears.