Abstract: Altered diabase is one of the common rock masses in engineering construction in western China, and its alteration degree has a great influence on the characteristics of engineering rock mass.In this paper, the altered diabase of the Yingliangbao Hydropower Station under construction on Dadu River was taken as the research object, and the types of altered diabase were classified according to the macroscopic geological characteristics, rock mineralogical characteristics and mechanical test results, and the fuzzy mathematical theory was introduced to classify the degree of surrounding rock of altered diabase.The results show that a wide distribution of diabase alteration zones in the project area was formed under the complex geological tectonic action, and the altered diabase could be divided into six categories: mild-altered diabase, cataclastic diabase, cracked diabase, diabase breccia, schistose diabase and argillaceous diabase.With the deterioration of alteration degree, the fragmentation of diabase and the degree of clayification such as kaolinite and chlorite gradually intensified, so the corresponding mechanical strength gradually decreased.According to the alteration characteristics of diabase, the saturated uniaxial compressive strength R_c, cohesion c, integrity coefficient K_v, structural plane characteristic parameter J_c and alteration index k_s were selected as the basis for the classification of surrounding rock, and a fuzzy comprehensive evaluation method for the classification of altered diabase surrounding rock was established, and the correspondence between rock type and surrounding rock grade was obtained.The feasibility and rationality of the method were verified by taking the multiple alteration diabase zones in two excavated diversion tunnels as an example.