The invention discloses a hydrodynamic retarder. The hydrodynamic retarder comprises a case (1) and a transmission main shaft (2), a front rotating impeller (3) and a rear rotating impeller (4) are arranged on the transmission main shaft (2), a braking pressure regulating switch, a gear pump and a water pump are arranged on the case (1), the gear pump is connected with the transmission main shaft, the water pump is connected with the gear pump, the front rotating impeller (3) is connected with the transmission main shaft (2) through a driving wheel flange (5), the rear rotating impeller (4) is connected with the transmission main shaft (2) through a planetary gear set, and oil inlet holes (6) and oil outlet holes (7) are uniformly arranged on the front rotating impeller (3). The hydrodynamic retarder has the advantages that the structural design is reasonable, the braking efficiency is high, the reliability is high, simultaneously the manufacturing is simple, the manufacturing cost is low, and the service life is long.