The invention discloses a flood-discharge tunnel-based combined energy dissipater synchronizing flood discharge and power generation. The energy dissipater comprises a water inlet tower, an underground main powerhouse, a contact hole, a main transformer chamber with a transformer, a vertical shaft, a tail water pipe, a tail water branch tunnel and a pressure-free tail water tunnel. The water inlet tower is provided with a water inlet, the water inlet tower guides water in a flood-discharge tunnel into the underground main powerhouse through a pressured water flow tunnel, a water turbine power generator is disposed in the underground main powerhouse, water flows generate power and dissipate energy through the water turbine power generator, current is guided to a ground step-up substation through the contact hole, the main transformer chamber and the vertical shaft, and the water flows flow into a downstream water channel through the tail water pipe, the tail water branch tunnel and the pressure-free tail water tunnel. The energy dissipater has the advantages that the energy of the flood-discharge water flows in the tunnel is converted into electric energy so as to achieve energy dissipation, energy of surplus water is fully utilized, the flow speed of the tail water acquired after surplus water power generation is quite low, surface rotation of downstream water is reduced greatly, downstream ecological environments can be improved effectively, and the dissipater is widely applicable to in-tunnel flood discharge of different flow rates.