Deformation control of surrounding rock in underground caverns with high side wall is always a huge challenge in underground engineering construction, and the deformation and failure of surrounding rock are prominent in the excavation process. Aiming at the deformation and failure of high side wall during excavation of extremely broken surrounding rockmass of a hydropower station, a method of controlling surrounding rock deformation with steel support is proposed. Based on numerical simulation and field monitoring, the effects of different types of steel support structures and surrounding rock support are quantitatively evaluated. The results show that: The circular steel pipe support structure is obviously superior to the composite Ⅰ-steel support structure. The deformation of the surrounding rock of the side wall is effectively controlled after the steel tube support is adopted, the range and depth of the plastic zone are obviously reduced, the stress concentration phenomenon is improved, and the stability of the surrounding rock is significantly improved. The results of field monitoring and numerical simulation agree with each other and verify the effect of steel support. The relevant results can provide reference for the deformation control of the surrounding rock of the underground cavern with high side wall under the condition of extremely fractured rockmass.