Low carbon, green, and high-efficiency ironmaking is an essential direction for developing ironmaking technology. Low-temperature reduction of ultra-fine iron ore powder can reduce reaction temperature and carbon dioxide emission, which has broad development prospects. The carbon monoxide reduction characteristics of ultrafine iron ore powder with different degrees of mechanical activation were studied by thermogravimetry. The reduction reaction mechanism was analyzed and discussed by KAS (Kissinger Akahira Sunose) method, kinetic model-fitting method, and activation energy judgment method. The results showed that mechanical activation is conducive to the reduction of iron ore powder with carbon monoxide. When the mechanical activation time was longer, the reaction characteristic temperature was lower. Before 120 min of mechanical activation time, the effect of mechanical activation time on reduction characteristic temperature was greater than that after 120 min. The reaction starts temperature of the sample powder with an activation time of 480 min was 523 K, which was 87 K lower than that of the non-activated sample powder. And the peak temperature of the reaction rate was 1108 K, which was 217 K lower than that of non-activated sample powder. When the mechanical activation time was longer, the reaction activation energy was lower, but the reaction mechanism had not changed, and the interfacial reaction had always been a restrictive step. The activation energy of iron ore powder after activation for 480 min was 31.23 kJ/mol lower than that without activation. With the increase in mechanical activation time, the reason why the activation energy of the reaction decreased was that the mechanical activation reduced the particle size of iron ore powder and the average particle size of iron ore powder, which was more conducive to the diffusion of carbon monoxide gas. Mechanical activation significantly impacted the step-by-step reaction process of carbon monoxide reduction in iron ore powder. For the unactivated iron ore powder, the step-by-step reaction level was not clear, but for the activated iron ore powder, the step-by-step reaction level was clear.