The invention provides a lower box body of a battery pack. The lower box body comprises an inner box body, a mounting plate and a module mounting beam, wherein the mounting plate is fixedly connected with the inner box body; a first turnup is arranged on the mounting plate and provided with a first through hole for fixing the lower box body of the battery pack on a vehicle body; the module mounting beam is mounted on the upper part inside the inner box body and has a fixing part for fixing the battery pack; the lower box body further comprises a reinforcing beam which is mounted on the upper part inside the inner box body and is connected with the module mounting beam. The reinforcing beam is arranged in the inner box body, so that the structural strength of the inner box body is increased under the joint action of the reinforcing beam and the mould mounting beam, and therefore, the structural strength of the lower box body and a battery pack mounting assembly for mounting the battery pack is further improved, and the shock resistance of the battery pack assembly is improved.