Abstract Loss-in-capacity of carbide slag in CO2 capture restricts the development of industrial wastes in calcium looping technology. In this work, a novel Mn/Mg-copromoted carbide slag was prepared using carbide slag, dolomite and trace Mn(NO3)2 additive. Experimental tests were carried out in the fixed-bed reactor to evaluate how the preparation and the reaction conditions influenced the CO2 capture performance of Mn/Mg-copromoted carbide slag during calcination/carbonation cycles. Results show that MgO diminishes the sintering of synthetic sorbents. The optimal Mn/Mg-copromoted carbide slag (mass ratio of CaO:MgO:MnO2 = 89:10:1) exhibits the highest CO2 capture capacity of 0.52 g/g after 10 cycles under the severe calcination condition (100 % CO2, 950 °C) and the wet carbonation condition (15 % CO2/20 % steam/N2), which is 1.7 times as high as that of untreated carbide slag. MnO2 positively affects the slow carbonation stage by enhancing the electron transfer between CaO and CO2. Observations of the morphology of Mn/Mg-copromoted carbide slag indicate that the stabilized CO2 capture performance is mainly attributed to porous structure, MgO as the skeleton and MnO2 as an electron-transfer promoter.