The recent increase in the seismic activity in the southern Sichuan Basin, China, is potentially related to shale gas hydraulic fracturing. However, the detailed mechanism of hydraulic fracturing requires further verification. In this study, high-resolution seismic profiles are used to reveal several large pre-existing faults in the basement. Based on the fully coupled poroelastic theory, we calculate the perturbing poroelastic stress field caused by hydraulic fracturing using a finite element model constrained by well-defined geological data, detailed injection data, and radar data. The results indicate that the small earthquakes are correlated with the distribution of the poroelastic stress, which is concentrated and extends a large distance along the weak stratum in the sediment beyond the fluid diffusion. Two moderate earthquakes are consistent with the increase of the Coulomb’s stress in spatial-temporal evolution; thus, they are likely triggered earthquakes resulting from the reactivation of deep pre-existing faults.