A synchronous technique for recycling plastics and detoxifying chromium slag by using steel slag waste heat. By means of heating steel slag and gasifying plastics, and then using a catalyst such as hazardous waste chromium slag, plastics are subjected to a high-temperature catalytic pyrolysis; under a steam gasification condition, the plastics are relatively thoroughly converted into a low-molecular high-temperature energy gas, avoiding the surface coking of the chromium slag. Meanwhile, the high-temperature energy gas is used for heating the chromium slag, and at the same time hexavalent chromium in the chromium slag is reduced into trivalent chromium, while the energy gas is being cooled, and CO2 and Cl in the energy gas is being absorbed by an alkaline substance in the chromium slag. In the process, energy is saved during detoxifying the chromium slag and cooling the steel slag, and at the same time, a high-quality energy gas is obtained.