The invention discloses an excavation structure and an excavation method for a large-span dome of a nuclear reactor cavity of an underground nuclear power station. According to the excavation structure and the excavation method, multiple rock walls at intervals are reserved in the dome, multiple closed construction spaces are formed in the dome by the rock walls, excavation construction is performed in the smaller construction spaces, so that the excavation span can be reduced temporarily, and the surrounding rock stress level and the deformation level in the construction process can be reduced; operation and construction are performed simultaneously in the multiple closed construction spaces, the inter-process interference is small, and the construction efficiency and the construction safety can be improved.