Regional stress states and fault geometries play important roles in earthquake rupture dynamics. Using the curved grid finite‐difference method, we conducted 3D spontaneous rupture simulations of the nonplanar Qujiang fault (QF) to investigate the rupture processes of the 1970 Tonghai earthquake and potential future earthquakes. A nonplanar fault model including topography was adopted and embedded in heterogeneous media. Regional stress orientations with an interval of 5° were tested, and various fault geometry models with different fault surface traces and fault dips were discussed. We also provided explanations for the unbroken northwestern segment of the QF and the seismic intensity anomaly in the Tonghai basin during the 1970 Tonghai event. Finally, we presented several future potential earthquake scenarios occurring on the QF at three nucleation locations. Our simulation results suggested that the maximum principal stress azimuth around the Tonghai area is N25°W and that the QF is most likely a complex dipping fault—the southeastern segment dips to the northeast, whereas the northwestern segment dips to the southwest. Our simulations also revealed that multiple explanations, including a regional stress rotation and an increase in the cohesion force, could account for the unbroken northwestern segment of the QF. Furthermore, the seismic intensity anomaly in the Tonghai basin can be explained by a low‐velocity structure. Future earthquake scenarios demonstrated that potential earthquakes nucleating at Eshan and Wujie in a complex dipping fault model could rupture the entire QF, thereby posing severe seismic risks to nearby regions. In contrast, when the nucleation point was located at Quxi, the rupture was constrained to the initial fault segment of the QF; however, caution should still be exercised in the Quxi area because this scenario produces a maximum intensity of VIII.