<p>Geological defects pose significant threats to the overall stability of high arch dams, making adequate reinforcement crucial for their safe operation. In this study, the stress characteristics and the overall stability of arch dam abutments are analyzed. The reinforcement mechanisms and design methods are investigated for two types of geological defects: karst caves and adverse structural planes. (1) For karst caves, the failure modes of arch dam abutments under arch thrust can be categorized into punching and punching-shear types. The expression of reinforcement forces for concrete replacement is proposed, along with the stress homogenization analysis. (2) For adverse structural planes, the effects of material properties and geometric characteristics on the shear stress distribution are investigated. The coefficient of reinforcement scope is defined, and a quantitative reinforcement design method is proposed. The proposed method has been validated in two projects, by assessing the effects of reinforcement measures on the two types of geological defects. The results indicate that (1) concrete replacement for karst caves leads to more homogeneous stress distribution and coordinated deformation in arch dam abutments; (2) the reinforcement measures designed by the proposed method for adverse structural planes can reduce the maximum stresses in the dam heel, toe, and abutments, decrease the transverse and longitudinal displacements, and significantly reduce the area of yield zone in abutments. The results offer a valuable reference for reinforcement of geological defects in arch dam abutments.</p>