Taking the gear transmission system of 2 MW horizontal shaft variable speed wind turbine as the research object,the reliability evaluation of the wind turbine gear with cracks was conducted.The random wind speed was used as the external load of the wind turbine gearbox,and the stress on the tooth root of the gear was calculated by the torque and tangential load of the gear,Weibull distribution was used for fitting.According to the relationship between fracture probability and the maximum stress of gear and Griffith theory,the model between fracture probability and crack was established.Then,the model of crack and load times was established based on Paris formula.Finally,by combining the two models and using the relationship between reliability and failure rate,a reliability model of wind turbine gear under random load was established.Taking the solar wheel in the wind turbine gearbox as an example,the results show that compared with the solar wheel whose initial reliability is more than 0.99,the reliability of the verified solar wheel declines sharply after the action of high load when the initial reliability is as low as 0.5,until it fails. Under the determined materials,the proposed model can better reflect the influence of crack extension and crack on reliability under different stress levels,and it can be well applied in engineering practice.