Abstract: The load on the arch dam mostly applies on the abutment rock mass of both sides of dam. Dam abutment is key part to maintain stability of arch dam and its stability is one of the important engineering geology factors in the process of arch dam construction. Two geological discontinuities with long persistence are found during the excavation of slope at left abutment of Wudongde hydropower station. According to the engineering geological conditions and data of feasibility study, in combination with special exploration and exposure of excavation surface, the distribution of discontinuities is obtained. The quantitative research about the physical composition and character of infillings of discontinuities is achieved through field investigation in detail and geophysical exploration test. Based on the results, the adverse impact of discontinuities to the stability of abutment is researched through engineering geological evaluation and numerical analysis. The results show that these discontinuities are two tectonic faults formed in the earlier stage and superimpose the modification of dissolution later. It is a typical phenomenon in limestone zone. Most of the infillings exist with calcium cementation. Partial infillings are poorer because of dissolution. Thus, the distribution of soft material of infillings is discontinuous. Hard rocks contact each other directly. The discontinuities have no impact on the abutment stability, but may cause the stress concentration in abutment rock mass partly. The developed approach is scientific and comprehensive. It can be applied to study the engineering geological characteristics of discontinuities and evaluate their impact to the stability of abutment rock mass in the process of dam construction. It also provides theoretical and practical significance to the survey and design of arch dam project.