The invention discloses a dam drainage structure which comprises a surface drainage hole, a surface drainage hole gate well, a bottom evacuation hole, a bottom evacuation hole gate well and a bottom water reflow tunnel. The surface drainage hole and the surface drainage hole gate well are formed in the upper portion of a dam, the bottom evacuation hole and the bottom evacuation hole gate well are formed at the bottom of the dam, and the bottom water reflow tunnel is communicated with the bottom evacuation hole gate well. A vertical well penetrating the bottom water reflow tunnel vertically is formed at the downstream of the bottom water reflow tunnel, and the vertical shaft is communicated with the surface drainage hole. By sufficiently utilizing limited building arrangement space, flood drainage and evacuation are realized; the surface drainage hole and the bottom evacuation hole share the same water reflow tunnel, so that the drainage structures of the surface drainage hole and the bottom evacuation hole do not have to be arranged separately, and excavating workload, supporting workload and occupation space are reduced.