Abstract: The main granite mass surrounding the underground cavern groups of Dagangshan hydropower station have many diabase dikes ().Their types, trends, shapes and scales are different. They run through and interpenetrate the granite body (4-12). These dikes have broken rocks, low strength, long adhesion fault along the boundaries. They become remarkable weak structure zones of the entire underground caverns, and bring greater difficulties and risks to the excavation and support. Investigations are carried to clearly understand the development characteristics and influence scope of diabase dikes for better guiding in-site construction. The investigations include the following works. Firstly, dike strikes, extension sizes, morphologic features, internal structure and material composition in different elevations are examined in detail. Then, the regional geological structure and evaluative rule are applied to analyze the diabase disks' invasion and the effect process of magmatic hydrothermal activity. Finally, based on systematic geological survey and evaluative rule analysis, the stability of dominant diabase dikes are evaluated reasonably and some significant supporting methods are recommended. In addition, a useful discussion is given about material sectional characteristics of diabase dikes and association between alteration-degree and forming sequence.