To make a comparison between coal gasification in molten blast furnace slag (MBFS) in different ambience and choose an appropriate agent to recover BF slag’s waste heat entirely, coal gasification with steam and CO2 in molten blast furnace slags was studied by isothermal thermo-gravimetric analysis. The effects of temperature and addition of MBFS were studied. Carbon conversion and reaction rate increased with increasing temperature and MBFS. Volumetric model (VM), shrinking core model (SCM), and diffusion model (DM) were applied to describe the coal gasification behavior of FX coal. The most appropriate model describing the coal gasification was SCM in steam ambience and VM in CO2 ambience, respectively. The reaction rate constant k(T) in CO2 ambience is greater than that in steam ambience, which means the gasification reactivity of coal in CO2 ambience is better than that in steam ambience. BF slag can effectively reduce the activation energy EA of coal gasification reaction in different ambiences. But, the difference of activation energies is not large in different ambiences. Based on the results of kinetic analysis including k(T) and EA calculated by the established model, CO2 was chosen to be the most appropriate agent.