‘Replacing rolling with sliding’ is the primary way to solve the large-scale wind turbine gearbox (WTG), reduce costs, and increase efficiency. Due to the fatigue reliability problem of sliding-bearing gearboxes in large-megawatt wind turbines, the 6.2 MW WTG is taken as the research object. Internal and external excitations of gears, random loads, electromagnetic characteristics and sliding bearings are comprehensively considered. The electromechanical coupling dynamics model of the WTG is built under random wind speed. The dynamic characteristics of the transmission components are studied. The strength degradation process of the transmission components throughout their life cycle is characterized using fracture mechanics theory. Based on reliability design theory and fuzzy reliability theory, a time-varying reliability model of a sliding-bearing WTG considering the fuzziness of allowable wear is established. The influences of strength degradation and sliding bearings on WTG’s time-varying reliability and sensitivity are explored. The research lays the foundation for the design and operation & maintenance of sliding-bearing WTGs.