The invention discloses a steel slag-based activated carbon for degrading formaldehyde and a preparation method of the steel slag-based activated carbon, and belongs to the field of metallurgical solid waste resource utilization. The steel slag-based activated carbon includes steel slag, a steel slag grinding aid, hydrochloric acid, activated carbon, absolute ethanol and water, wherein the steel slag is one or more of drum slag, desulfurization slag and hot stew slag, and the steel slag grinding aid is a mixture of ethylene glycol, triethanolamine and absolute ethanol. The invention solves theproblems that although the formaldehyde degradation performance of activated carbon is improved through modification of the activated carbon by a metal oxide in the prior art, the problem of increaseof the preparation cost is caused, reduces the production cost of the modified activated carbon by 40% to 50%, and improves the market competitiveness and application range of the modified activatedcarbon. At the same time, compared with a current steel slag modified activated carbon technology in which steel slag is physically mixed with activated carbon, the invention utilizes the combinationof a grinding aid technology and a chemical compounding technology to greatly improve the performance and stability of the steel slag-based activated carbon.