Abstract: Steel slag–peanut shell-based activated carbon was prepared using ultrafine steel slag powder and peanut shells through microwave processing. The response surface method was used to evaluate the effects of microwave power, impregnation ratio, steel slag content, and steel slag particle size on the rate of the adsorption of formaldehyde gas by the prepared activated carbon. Subsequently, optimum parameters were calculated for the preparation of activated carbon with the maximum rate of adsorption for formaldehyde gas adsorption. Finally, the activated carbon was characterized by an X-ray infrared spectrometer, field emission scanning electron microscope, and specific surface area and pore size analyzer. Results revealed that the activated carbon prepared using 530 W of microwave power, steel slag powder corresponding to a mesh size 1160, steel slag content equal to 10.8%, and impregnation ratio of 1.25 has the highest formaldehyde adsorption rate. According to the established regression model, the theoretical adsorption rate of formaldehyde gas will be 94.96% under the above optimal preparation conditions. Thus, the prepared activated carbon had a formaldehyde adsorption rate of 94.14%, which is within a 5% error range of the adsorption rate estimated by our regression model for the same conditions. We further demonstrated that our response curve model can predict the adsorption rate of the activated carbon prepared by this process efficiently and that it is feasible to optimize the preparation of activated carbon by the response surface method. Furthermore, the regression analysis further reveals that the degree of influence of the four factors related to this method of preparing activated carbon on the rate of formaldehyde gas adsorption is in the following order, from large to small: microwave power, steel slag content, impregnation ratio, and steel slag fineness. The mutual interaction of the four influencing factors on the formaldehyde gas adsorption rate can be intuitively observed through the three-dimensional response surface graph. Pore structure analysis of the activated carbon prepared using the optimal preparation conditions revealed that it has an H3-type hysteresis loop and a flat-panel slot-like structure. The pore size distribution is uneven, with predominant micropores and small-sized mesopores. Fourier-transform infrared spectroscopy analysis showed that after adding steel slag for modification, the activated carbon had more acidic functional groups, which is beneficial to the adsorption of formaldehyde. Morphological analysis reveals that the layered structure of the activated carbon is clear and that adding a small amount of steel slag is beneficial to improve the rate of pulverization.