Abstract: The excavation-induced relaxation of dam base surface can enormously affect the global stability of 300 m level-high arch dams. Unfortunately, for Baihetan arch dam, serious excavation-induced relaxation and structural plane dislocation are discovered in the left slope when the dam base surface is excavated to the elevation of 628 m. Based on the deformation reinforcement theory, the unbalanced force is used as a quantitative index to evaluate the degree of excavation-induced relaxation of dam base surface. Then, the theoretical basis of rock cracking and relaxation based on the unbalanced force can be analyzed. Simultaneously, the accuracy of using the unbalanced force to evaluate excavation-induced relaxation is illustrated by experiments and numerical simulations. Employing the 3D nonlinear FEM programme TFINE, the excavation-induced relaxation evolution process of the arch dam's left base surface of Baihetan, particularly the shear moving of the structural plane, is elaborated. Three excavation situations are investigated respectively: excavation without foundation treatments, excavation with reserved protective layer and excavation with anchoring. The simulated results show that the unbalanced forces and yield zones of structural planes are concentrated near the intersection zones and outcrops of structural planes, which fit the observations of relaxation of dam base surface and dislocation of structural planes well. LS331 and LS3318 are more dangerous than the other structural planes owing to larger unbalanced force during excavation. The unbalanced force near the scarp bottom obviously increases during excavation of the scarp, so the gradient of the scarp should be slackened. Relatively speaking, reserving the protective layer and setting anchoring have little influence on controlling the excavation-induced relaxation.