In this study, the 37-day time lag was estimated by cross-correlating the daily number of earthquakes and the daily water level data from 1 March 2013 to 31 October 2019 in the Longtan reservoir, Guangxi Zhuang Autonomous Region. Based on the surrogate data test, we confirmed the 37-day time lag was reliable. The 37-day time lag indicates reservoir-induced enhances seismic activity rapidly after the water level reaches peak value, which may suggest the fluid effect of reservoir water is mainly confined within the reservoir area at present. With the time lag, the depth distribution of the earthquakes, and the solutions of pore pressue diffusion equations derived under periodic boundary condition, the vertical hydraulic diffusivity were estimated to be D
/s and D
/s on the condition that the pore pressure diffusion uncoupled and coupled the stress, respectively. The latter is more reasonable physically, which may indicate the coupling between diffusion of the pore pressure and stress is the main reason for inducing the earthquakes in the Longtan reservoir at present stage.