The rock mass alteration zone developed in the high sidewall and rock wall crane beam of the Yangfanggou large underground powerhouse seriously affects the stability of surrounding rocks and structures. Firstly, the macrogeological conditions, monitoring deformation characteristics, acoustic wave detection data and rock mechanics tests of the surrounding rock affected by the alteration rock were analyzed, and a method of rock mass quality zoning and comprehensive evaluation of rock mass mechanical parameters based on the rock Pwave velocity was proposed. Simultaneously, the spatial division and evolution characteristics of the unfavorable geological body were also identified, which can combine the detection results with numerical simulation technology quickly, dynamically and quantitatively. Then, by constructing a reliable numerical model, quantitative analysis and evaluation of the adverse effects of the rock alteration zone were carried out, and the safety warning standard during construction was proposed. The results provide support for reasonable construction plans and active protection and reinforcement measures.