The main dam at the upper reservoir of Tianhuanping pumped storage power station is a rockfill dam with an asphalt concrete impervious lining on the upstream face. It is constructed on a non-homogeneous fully weathered rock foundation. In this paper, we present the case study on the treatment for this non-homogeneous fully weathered rock dam foundation. The treatment includes the partial excavation of the fully weathered rock in different depths with respect to the dam deformation, and the enlargement of the transition curvature at the junctions of the inclined dam face and the reservoir floor, as well as the reinforcement for the asphalt concrete impervious lining with a layer of polyester mesh at the parts where tensile strains of the lining are large. A three-dimensional (3D) finite element analysis is carried out for the main dam to provide the basis for the proposed compound treatment method. To account for the effect of the intermediate principal stressσ2 on the deformation and strength of soils, a simple constitutive soil model is proposed for the finite element (FE) analysis. So far, this project has operated well for more than 5 years without any maintenance for the main dam’s asphalt concrete impervious lining, illustrating the success of the treatment for the fully weathered rock dam foundation.