Landslides in the reservoir area are a key issue of national hydropower engineering safety and geological disaster prevention and control. During the water storage cycle, the water level in the reservoir area changes significantly, and water level fluctuations have a strong triggering effect on landslides in the reservoir area. This article takes the Baihetan hydropower station reservoir area as the research area, uses Small-Baseline Subset InSAR technology, combined with Sentinel-1 A data from the lifting rail, to monitor deformation in the early, middle, and late stages of water storage in the research area, and studies the impact of water level changes in the reservoir area on landslide deformation. The results showed that: (1) deformation monitoring was carried out in the study area using InSAR technology, and 11 unstable slopes were detected, with the maximum deformation reaching 0.14 m. Among them, 4 were known landslides, and deformation monitoring was carried out on typical landslides to study the impact of water level changes in the reservoir area on deformation; (2) Taking the cross mountain landslide and Qiaojia County landslide as examples, this study investigates the correlation between water level fluctuations and landslide deformation. Through Pearson correlation coefficient research, it is found that there is a strong correlation between significant changes in water level and landslide deformation rate. The correlation between the two is highest in the middle stage of water storage, and the impact of rising water level in the reservoir area on landslide deformation is greater than that of falling water level; (3) Taking a typical landslide as an example, a lag study was conducted on the water level and landslide deformation in the reservoir area through wavelet analysis. It was found that there is a lag period of about three to four months between the landslide deformation and water level fluctuations in the reservoir area.