This work investigates the impact of the magnitude of cycling voltage on the fatigue characteristics of 40 nm-thick AlScN ferroelectric thin film. The fatigue rate and the rejuvenation of remanent polarization vary with the cycling voltage. The primary fatigue mechanism is identified to be the interfacial layer formation and domain wall pinning at high and low cycling voltages, respectively. Additionally, annealing the film under the NH3 atmosphere decreases the fatigue rate and improves endurance by eliminating impurities in the film. The amount of trapped charges at the interface also decreases after NH3 annealing, leading to a reduction in leakage current. Furthermore, the ferroelectric performance of the AlScN film is not degraded after the thermal annealing at 900 °C under the NH3 environment, suggesting its robustness against the severe thermal budget. It is concluded that NH3 annealing is a promising method to address the reliability issue of the AlScN film.