The invention belongs to the field of surrounding rock supporting construction methods, and particularly relates to a method for supporting surrounding rock of a diversion tunnel. The method for supporting the surrounding rock of the diversion tunnel aims to solve the problems that a small-section extra-long diversion tunnel is limited in construction space, and an originally-designed lining construction method is long in period and long in construction period. The method for supporting the surrounding rock of the diversion tunnel comprises the steps that S1, supports are arranged, specifically, profile steel is laid within the range of a top arch and side walls to serve as the supports; S2, anchor rods are arranged, specifically, the anchor rods are laid within the range of the top arch and the side walls, the anchor rods are arranged once every other truss in a supporting mode, and three anchor rods are arranged in each row; S3, longitudinal connecting ribs are laid, specifically, the connecting ribs are laid within the range of the top arch and the side walls, so that the connecting ribs are connected with the supports; S4, a reinforcing mesh is laid, specifically, the reinforcing mesh is laid within the range of the top arch and the side walls; S5, concrete is sprayed, specifically, the concrete is sprayed within the range of the top arch and the side walls; and S6, secondary lining is conducted, specifically, plain concrete is poured within the range of the side walls. Tunnel lining is changed into a channel form, and original trolley lining is changed into formwork lining, so that the construction period is shortened, and benefits are increased.