The invention relates to reduction smelting slag and application of the reduction smelting slag to reduction smelting of coarse antimony oxide, and belongs to the field of nonferrous metallurgy. The designed reduction smelting slag comprises, by mass, 20-40 parts of SiO2, 20-40 parts of FeO, 5-20 parts of CaO and 5-25 parts of Na2O. During reduction smelting of the coarse antimony oxide, the set reduction smelting slag is supplemented, the temperature ranging from 900 DEG C to 1,200 DEG C is kept for 30-120 min, coarse antimony, glass slag, smoke and flue gas can be generated, the coarse antimony and the slag are independently discharged after settling separation, the smoke is subjected to powder returning treatment, and the flue gas is exhausted after being purified. The design technology has the beneficial effects that the smelting temperature is low, the antimony content of the slag is small, the antimony direct recovery rate is high, and the powder returning amount is small; and large-scale industrial application is facilitated.