Wind power converter (WPC) is a key part of a wind power unit which delivers electric energy to power grid. Because of a large number of semiconductors, WPC has a high failure rate. This paper proposes a method to accurately evaluate the reliability of WPC, which is crucial for the design and maintenance of wind turbines. Firstly, the index of effective temperature (ET) is presented to quantify the effects of temperature and humidity on the semiconductor operation. A novel method is proposed to evaluate the lifetime and calculate the aging failure rates of the semiconductors considering the fluctuations of ET. Secondly, the failure mode and effect analysis (FMEA) of WPC is investigated based on the topology and control scheme. The conventional two-state reliability model of the WPC is extended to the multi-state reliability model where the partial working state under the fault-tolerant control scheme is allowed. Finally, a reliability evaluation framework is established to calculate the parameters of the WPC reliability model considering the variable failure rates and repair activities of semiconductors. Case studies are designed to verfify the proposed method using a practical wind turbine.