<p>For the cross-fault hydraulic tunnel, rationally simulating the surrounding rock and rock-fault interface, revealing its destabilization risk areas, and evaluating its safety degree are of great significance for its engineering design and construction. Based on the Zienkiewicz-Panda yield criterion and the Drucker-Prager plastic potential function, an elastic-plastic damage constitutive model of the surrounding rock was established, and its rationality was proved. The failure modes of the rock-fault interface were divided into normal disengagement damage, abrasion damage, and snip damage. The peak shear strengths of the structural surface under different shear failure modes were deduced, which were combined with the Desai thin-layer contact element model to form the numerical simulation method of the rock-fault interface. Finally, an evaluation index called the element dangerous coefficient (EDC) was proposed, which is capable of evaluating the element’s safety degree in the whole deformation and damage process. The proposed methods were applied to a cross-fault hydraulic tunnel, and some conclusions were obtained as follows. (1) The value of the EDC gradually decreases from the excavation surface to the interior rock mass, which indicates that the danger degree of the rock mass is gradually decreasing. (2) The rock-fault interface is the weak area, and the contact surface near the excavation surface is disengaged after the excavation, and the EDC value reaches 2.0. (3) The EDC index is capable of quantitatively evaluating the element’s hazard level in the elastic and plastic zones at the same time and giving the element’s hazard coefficient.</p>