The present invention discloses a method for treating heavy metal sludge by using stainless steel molten slag. The method comprises drying crushing, material matching mixing, pressing molding, drying, tank bottom material spreading, smelting reduction, gas purification, molten slag treating, metal recovery and other steps, and specifically comprises that: after heavy metal sludge is subjected to pre-treatment, a certain proportion of ingredients are matched, and the sludge agglomerate is adopted as the bottom spreading material and is placed into a stainless steel molten slag tank to carry out a harmless treatment on the sludge agglomerate. According to the present invention, the characteristics of high production amount, high temperature and rich sensible heat resource of the stainless steel molten slag are adopted, and the stainless steel molten slag is adopted to treat the heavy metal sludge, such that the harmless treatment on the hazardous components and the resource on the valuable material are achieved, the method is the new attempt crossing the industry, and the waste heat resource can be effectively utilized so as to achieve the waste treatment adopting the waste.