Wind power converter (WPC) is a key component of wind energy conversion system (WECS). It is responsible for the important functions of wind power conversion and grid connection. Its failure will cause the shutdown of the entire WECS so that its reliability has a great impact on the reliability of the entire WECS and even the entire wind farm. Due to the harsh operating environment of offshore wind farms and the difficulty in repairing and replacing faulty components, WPC has a more serious impact on the overall reliability of offshore wind farms. Based on the mechanism of the WPC failure, a reliability model of the WECS in offshore wind farms is established, which considers the WPC failures. Based on the established reliability model, this paper further considers the wind turbine wake effect and the failure of the wind farm’s collection system to evaluate the reliability of the offshore wind farms. This paper studies the impact of WPC failure on the overall reliability of offshore wind farms and conducts a sensitivity analysis of the main factors affecting the reliability of WPC and offshore wind farm. The results of case studies show that the WPC failure has a great impact on the overall offshore wind farm’s reliability. After considering the impact of WPC failures, the capacity factor of offshore wind farms will be significantly reduced.