The invention discloses a bottom-sudden-expansion gyrating shaft well flood discharging tunnel. The flood discharging tunnel comprises a lock chamber, an upper flat section, a volute chamber, a contraction section, a shaft well vertical section, a slope section and a flood discharging tunnel body. A sudden-expansion vertical section is disposed on the vertical lower portion of the shaft well straight section; the axes of the sudden-expansion vertical section and the shaft well vertical section are located on the same straight line, and the diameter of the sudden-expansion vertical section is larger than that of the shaft well vertical section; the sudden-expansion vertical section is connected with the shaft well vertical section through a connecting section, and the connecting section isin smooth transition connection with the shaft well vertical section; an outlet of the shaft well vertical section is correspondingly connected with an inlet of the connecting section; an outlet of the connecting section is correspondingly connected with an inlet of the sudden-expansion vertical section; the bottom of the sudden-expansion vertical section is sealed; an outlet is formed in the sidewall of the sudden-expansion vertical section; and an inlet of the slope section is correspondingly connected with the outlet of the sudden-expansion vertical section so that the slope section communicates with the sudden-expansion vertical section. The flood discharging capacity of a gyrating shaft well is improved, the energy dissipation efficiency and effect are improved, and the flood discharging burden of an existing gyrating shaft well flood discharging tunnel is reduced.