The invention belongs to the technical field of iron and steel metallurgy and in particular relates to a low-cost slag washing method. The low-cost slag washing method comprises the following steps: producing synthetic slag for slag washing in a steel ladle, obtained by casting molten steel, by utilizing refining slag and a deoxidant; then utilizing the produced solid-state synthetic slag with relatively high temperature for slag washing operation of a steel tapping process of the next heat. The method provided by the invention has a simple technology and is low in cost; the consumption of slag production materials is less and the heat of the refining slag is recycled; smooth production is not influenced; compared with cyclic utilization of heat-state refining slag, the low-cost slag washing method has remarkable advantages. Sulfur is effectively removed.