Due to the complex geological processes of Qinghai-Tibet Plateau, numerous deposits, especially the large-scale ancient landslide deposits, are characteristic features of the valleys incised in southwestern China. Intense water level fluctuations since 2011 in Maoergai Reservoir, China, registered the reactivation of Xierguazi ancient landslide, and presented a significant risk to neighboring facilities. Based on detailed field survey and drilling exploration, the landslide was divided into Zone A and Zone B, and other characterizations of landslide were studied as well. To precisely measure the extent of landslide displacement during filling and drawdown stage, surface displacement monitoring system was deployed on the landslide. The monitoring analyses data reveal that reservoir fluctuation is the dominant factor influencing landslide displacement, especially during drawdown stage. Moreover, a future sliding is anticipated in Zone A, while a creep had already existed in Zone B. A reservoir regulation was then established using the lead-lag correlation between reservoir fluctuation and landslide displacement and landslide stability analysis. In the end, the follow-up deformation monitoring demonstrates that the reservoir regulation controlled the landslide effectively. Landslide control by reservoir regulation in Maoergai can serve as a case study for other settlements involved in similar construction activities.