The invention discloses a hollow guiding excavation structure and an excavation method for an ultra-long span dome of an underground nuclear reactor cavity. The structure comprises a hollow pilot tunnel under the center of the dome, wherein the hollow pilot tunnel extends to the horizontal plane of the arch springing of the dome profile from the middle of the rock face of the dome profile along the vertical direction; an excavation exposing face is arranged between the hollow pilot tunnel and the cavity rock body; a first construction channel and a second construction channel are formed between the rock face of the dome profile and the hollow pilot tunnel; the first construction channel extends towards the hollow pilot tunnel from one side of the outer edge of the dome along the diameter direction; the second construction channel is provided with a transverse channel which is positioned in the same direction with the first construction channel; the transverse channel extends towards the hollow pilot tunnel from the other side of the outer edge of the dome along the diameter direction; and the first construction channel and the transverse channel are communicated in the hollow pilot tunnel. According to the hollow guiding excavation structure, the excavation span can be temporarily reduced, and the surrounding rock stress level and deformation level in the constructing process can be reduced; and the structure has small interference among processes, and the construction efficiency and construction safety can be improved.