The invention discloses a bottom punching mechanism for a fire protection main unit shell. The bottom punching mechanism comprises a rack, wherein two insertion slots are formed in the front and back sides of the top surface of the rack; a first clamping plate and a second clamping plate which are "L"-shaped and oppositely arranged are inserted into the insertion slots; a plurality of vertical discharging through slots are formed in the top surface of the rack between the two insertion slots; a punching block body is inserted into the shell to be machined; a plurality of punching heads are formed on the bottom surface of the punching block body; the punching block body is fixed on the bottom surface of a lower pressing plate; two vertical connection plates are fixed at the left and right ends of the top surface of the rack; the top ends of the connection plates are fixedly connected with a top plate; guide rods are inserted on the top plate; the lower ends of the guide rods are fixedly connected to the middle part of the top surface of the lower pressing plate; impact cams are arranged above two sides of the lower pressing plate; transmission tooth rings are formed on the arc-shaped walls, opposite to the protruded parts of the impact cams, of the impact cams; and the transmission tooth rings are connected with rotating gears in a meshing manner. The bottom punching mechanism is applicable to fixed and firm clamping of the fire protection main unit shells with different sizes, and is high in punching efficiency.