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High arch dams present nonsymmetry problems, both on topological and geological conditions at dam site, and are inevitable in many projects. Their influence is unfavorable to the performance of the dam-foundation system, and the mechanism needs to be studied to find proper treatments in giant rock engineering. Baihetan Dam is a high arch dam with typical nonsymmetry problems, and is studied in this paper as an example. A geomechanical model test was conducted focusing on features of deformation and failure phenomena, and a numerical calculation based on a nonlinear finite element method (FEM) was performed on it. Deformation reinforcement theory was particularly involved in the FEM calculation to quantize and describe the instability status of structure and proved to be reliable. It is found that the nonsymmetrical profile causes the allocation of arch thrust to be asymmetrical, leading to an approximately equivalent rotation of asymmetric axis of deformation. This phenomenon contributes to causing local regions of the dam to be under an extra large tension or compression in the overload process. The asymmetrical geological conditions, characterized by many developed tectonic surfaces in the left abutment, are revealed to threaten the stability of the rock mass. Global safety factors of the dam decrease as a consequence, when comparing the results with other dams of similar dam height. In the end, the revised definition of Lombardi's slenderness coefficient was proposed considering nonsymmetry of the profile. [ABSTRACT FROM AUTHOR]