Under the background of the “dual carbon” goals and sustainable energy development, the construction of pumped storage power stations is entering a stage of high-quality and rapid growth. However, this process has also brought about a large number of deeply buried underground cavern groups characterized by complex geological conditions, high in-situ stress, large spans, and high construction difficulty. Accurately, safely, and efficiently analyzing the stability of surrounding rock in such complex geological settings is a key technical support for the rational design, safe construction, and stable operation of pumped storage power stations. The underground powerhouse cavern group of the Lianghekou hybrid pumped storage power station on the Yalong River is notable for its massive scale, high cavern density, and complex construction conditions, with numerous interrelated factors affecting rock mass stability. In this study, a three-dimensional numerical investigation of the surrounding rock stability of this large underground cavern group is conducted based on the discrete element method, which accounts for the discontinuous deformation characteristics of structural planes. A 3D numerical model of the cavern group is established using the large-scale discrete element software 3DEC, and the in-situ stress field is back-calculated using field-measured borehole deformation data. The model comprehensively considers unfavorable structural features such as bedding planes and extrusion zones, as well as different support schemes. The stability analysis is carried out from three perspectives: overall stability of the cavern group, evaluation of support schemes, and local stability of key areas, providing technical references for engineering design and construction.