In the design of wind turbine gearboxes, the most important objective is to improve the durability to guarantee a service life of more than 20 years. This work investigates how external loads caused by wind fluctuation influence both the load distribution over the gear tooth flank and the planet load sharing. A whole system model is developed to analyze a wind turbine gearbox (WTG) that consists of planetary gearsets. Two models for different design loads are employed to quantify how external loads acting on the input shaft of the WTG affect the load distribution of the gears and the load sharing among the planets under quasi-static conditions. One model considers only the torque for the design load, whereas the other model also considers non-torque loads. For two models, the results for the gear mesh misalignment, contact pattern, load distribution, and load sharing are different, and this leads to different gear safety factors. Therefore, the results indicate that it is appropriate to consider the non-torque loads in addition to the torque as the design load for a WTG, and that this is very important to accurately determine the design load that guarantee the service life of a WTG.