The Yangtze-to-Hanjiang River Diversion Project is the first large-scale project under construction subsequent to the South-to-North Water Diversion Project. Its diversion tunnel intersects the active creep Tongcheng River fault. Understanding the in-situ stress characteristics of the active fault is crucial for evaluating the project’s stability. The in-situ stress near the complex Tongcheng River active fault was measured, and the fault stability was analyzed to reveal the current geostress state and the critical conditions for fault slip instability. In-situ hydrofracturing tests were conducted in two nearly 1,000-meter-deep boreholes in the vicinity of the Tongcheng River active fault. The results indicate that the current stress state within the measurement depth is divided into two types. Specifically, the spatial principal stress state is hypothesized to transition from a composite (reverse and strike-slip) or reverse type to a normal type at approximately 900±20 meters depth. This suggests that the in-situ stress in the near field is influenced by intersecting faults. The direction of the maximum horizontal principal stress shifts from NW to NWW with increasing borehole depth, which aligns with the dynamic characteristics of active fault movement, the left-slip mechanism of intersecting faults, and the focal mechanism solutions. Finally, based on the measurement data and in line with the Mohr-Coulomb friction sliding criteria and Byerlee’s law, the stability of the active faults was analyzed. Findings reveal that the overall stress accumulation near the Tongcheng River active fault is relatively low and has not reached levels that would induce instability, suggesting that the crust remains relatively stable. These findings provide essential geological and mechanical data for assessing the stability of the Yangtze-to-Hanjiang River Diversion Project near the Tongcheng River active fault zone and offer valuable insights for engineering design across active faults.