As a critical mechanical component of a wind turbine, gearbox failures mainly attribute to cumulative fatigue. Therefore, understanding and quantifying fatigue impacts to wind turbine operational life is important for its design optimization. According to ISO 6336, gearbox service life under variable load can be calculated and the damage due to cumulative fatigue can be quantified. While wind turbine is exposed to random wind speed with certain statistical distribution and different turbulence scales, which becomes the main impacts of transient gearbox load variation. By introducing different turbulence scale this paper quantifies gearbox load spectrum variation and cumulative fatigue damage to predict gearbox service life. The result obtained in this paper can be used for offshore wind turbine gearbox design optimization.