The invention discloses a straw breaking device which comprises a base, a supporting rod, a smashing bin and a discharging bin. The smashing bin is arranged above the end, away from the supporting rod, of the upper surface of the base, a platen is arranged above the upper end face of the smashing bin, the discharging bin is located below the lower end face of the smashing bin, the discharging binis connected with the smashing bin through a connecting assembly, a cutterhead is arranged at the lower end inside the smashing bin, a tool assembly is installed at the upper surface of the cutterhead, a supporting base is arranged in the center of the lower surface of the cutterhead, a rotating shaft is inserted into the supporting base, the upper end of the supporting rod is fixedly connected with the center of the lower surface of the cutterhead, a commutator is arranged at the upper surface of the base, the lower end of the rotating shaft is connected to the output end of the commutator, and the input end of the commutator is connected with a motor. In the using process, cake-shaped straw blocks are rapidly crushed, so that the crushing efficiency is ensured, the length of crushed straw monomers is small, the good crushing effect is ensured, and the phenomenon that the crushing device is blocked is avoided.