The invention discloses a lower slag jacking machine used for a vacuum-smelting reduction device. The lower slag jacking machine comprises a bottom plate, wherein the lower side wall of the bottom plate is equipped with four rollers; the upper side wall of the bottom plate is equipped with a servo oil cylinder; the drive end of the servo oil cylinder is connected with a receiving slot; a funnel isinserted into the upper end of the receiving slot; the funnel communicates with the inner part of the receiving slot; an inclined plate is arranged inside the receiving slot; the inner bottom of thereceiving slot is equipped with a rotary motor; the drive end of the rotary motor is connected with a rotary rod; the upper end of the rotary rod penetrates through the inclined plate and extends to the outer part of the funnel; two supporting plates and a storage box are further arranged on the upper side wall of the bottom plate; and the upper ends of the two supporting plates are equipped witha fixed plate. The lower slag jacking machine realizes automatic slag discharge work of magnesium slag, saves labor power, improves working efficiency, performs adsorption treatment on flue gas and dust in the magnesium slag, protects the environment, is simple in structure, and is convenient to popularize.