The factor of slope’s stability safety is influenced by various factors, among which groundwater level processed by different methods has great influence on the stability of slope. Due to the complexity of actual engineering problems, we generally need to obtain new groundwater level based on some assumptions. According to limit equilibrium theory, we compared and analyzed the influences of two different methods of processing groundwater line on the safety factor, and discussed the critical groundwater level corresponding to different reservoir water levels. An ancient landslide at Wujiang was taken as a case study. Result shows that 1) under the assumption that the reservoir water level is directly connected with the original groundwater level, the safety factor of landslide increases slowly and then decreases gradually with water level rising; 2) in consideration of groundwater level fluctuation, the safety factor of landslide tends to decrease with the increase of storage water level, indicating that the safety factor is conservative; 3) the factor of proportionality of critical groundwater level in the presence of different reservoir water level is between 0 and 1.