The invention discloses a main shell device for a hydrodynamic retarder. The main shell device comprises a square shell, wherein a spherical shell is arranged on one side of the square shell, a through hole is formed in the top of the spherical shell, a connecting boss is arranged on the top of the through hole, and an air inlet and an air outlet are formed in two opposite corners of the top of the square shell. By utilizing a spherical design, the stress of a unit area is relatively small, and the performance intensity is not influenced; meanwhile, the reinforcing ribs are additionally arranged outside of the wall, so that the strength of the square shell is enhanced, the pressure of the unit area is reduced, and meanwhile, the heat dissipation effect of the retarder is improved.