The invention provides an excavation relaxation treatment method for a rock mass dam foundation with a multi-layered structure. The excavation relaxation treatment method adopts an optimal dam foundation excavation relaxation treatment technology based on ways of structural surface pre-reinforcement, advanced consolidation grouting, energy-dispersive blasting and hole-bottom detonation. Compared with a traditional protective layer stratified excavation method, the relaxation treatment method has the advantages that excavation on 3 layers needs not to be adopted, so that the construction disturbance is reduced, the construction progress is accelerated, and the construction period is shortened; and the excavation relaxation and damage control problems of a rock mass with a multi-layered structure are effectively controlled.