Water level fluctuations that occurs during filling-drawdown operations in man-made reservoirs can result in accelerations or reactivations (or even catastrophic failure) of large landslides. For disaster mitigation , a monitoring system comprising manual and automatic GPS systems, borehole inclinometers , and groundwater piezometers was installed to investigate the slope deformation behavior of a large reactivated landslide known as the Shuping landslide at the Three Gorges Reservoir, China. The long-term monitoring data (2003–2015) focused on the landslide deformation, which was triggered by water level fluctuations, is presented in this study. The cumulative displacement curves showed a step-like behavior, suggesting a lower base activity superimposed by phases of acceleration, which appeared during reservoir drawdown periods and rainy seasons. Based on statistical Pearson correlation analysis, the increase in landslide activity is dominated by reservoir water drawdown, rather than rainfall. The durations of the slide acceleration phases are much shorter than the reservoir water drawdown durations. Based on in-site piezometer monitoring data and numerical simulation, the potential causes are the hydraulic processes in the slope with much smaller groundwater drawdown velocities and hydraulic gradients in the initial stage of the reservoir water drawdown period.