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Using RMR surrounding rock classification method, the surrounding rock quality of underground main powerhouse of a hydropower station is evaluated with a small evaluation unit. The results of surrounding rock classification show that the surrounding rock of main powerhouse is mainly Grade III, and Grade II and IV surrounding rock develop intermittently with the depth of main powerhouse, and there is no Grade I or V surrounding rock distribution. Secondly, setting a smaller evaluation section is conducive to improve the accuracy of surrounding rock quality evaluation and better grasp the distribution of different grades of surrounding rock in the evaluation area.