The invention discloses a method for diluting copper and nickel concentrate smelting slag, and relates to a method for smelting copper and nickel concentrate by fire. The copper and nickel concentrate smelting slag is diluted and smelted by adopting a direct-current electric furnace. The method is characterized in that positive and negative electrodes of the direct-current diluting electric furnace for smelting are vertically inserted into a slag layer in the furnace from the upper part of the direct-current electric furnace to perform diluting and smelting. The method of the invention effectively simplifies the process of diluting the slag, reduces power consumption and prolongs the service life of the furnace; and the final slag contains about 0.45 percent of copper. The method meets the requirement of direct discharge, and fulfills the purposes of improving the diluting speed and saving the energy consumption.