Wanjiakouzi arch dam is constructed in a complex geological site, where a large number of faults, interlayer shear bands and karst zones exist in the dam foundation and abutments. The weak rocks have a bad effect on the stability of the arch dam. In this paper, the displacement distribution and stress field of the arch dam are analyzed using 3D nonlinear finite element method. The numerical results show that displacement between both left and right bank abutments is obviously asymmetric and there is greater compression and shear deformation along the faults under the thrust of the arch dam. Furthermore, the tensile stress at dam heel cannot meet the requirement of the design specifications. Therefore, it is necessary to reinforce the dam foundation and abutments to improve the ability to resist deformation of abutment. By taking such reinforcement measures as concrete plugs and consolidation grouting for weak zones, the asymmetric displacements between abutments are decreased significantly and the tensile stresses of dam heel meet the stability requirement.