Fatigue life is an important performance for wind turbine gearboxes. A fatigue damage calculation model for wind turbine gearboxes was proposed under random wind loads based on wind turbine global analysis, gearbox dynamic analysis and gear fatigue damage analysis. Firstly, the wind turbine global analysis was done to calculate a gearbox’s input torque and output rotating speed under random wind loads. Then, based on the acquired input torque and output rotating speed, each gear’s dynamic contact force and rotating speed were calculated with the gearbox dynamic analysis. Finally, the gear fatigue damage analysis was done, gear’s contact force was converted into loads causing gear’s bending and contact stresses, according to Miner linear damage accumulation theory, each gear’s bending and contact fatigue damages were calculated. In the discussion part, each gear’s bending fatigue damage and contact fatigue damage were calculated using the proposed model under wind conditions with the mean wind of 11.5m/s and 18m/s, and the turbulence density of 14%. The most dangerous one in all gears was found. The study results were meaningful for detecting wind turbine gearboxes’ failure sources.