The models of substituting limestone for lime in a converter in the slag-splashing furnace-protection stage and open blowing stage of steel making were established, and the high-temperature decomposition experiments of large particle-size limestone were carried out for different particle size under different heat transfer conditions. The decomposition conversion rate of limestone α was obtained by experiments, and through the analysis of suitable rate control steps, isothermal kinetic mechanism of large size limestone heating in the two stages were researched. The results show that the conversion of limestone decreases with the increase of particle size and increases with the rise of temperature. Interfacial chemical reaction controls the rate in slag-splashing furnace-protection stage, the decomposition model is random nucleation and subsequent growth pattern, and its mechanism is in line with the Avrami-Erofeev equation. The most probable integral fuction of limestone decomposition is G(α)=[-ln(1-α)]ⁿ, n=3/4. Product layer heat transfer and CO₂ diffusion control the rate in the open blowing stage. When the temperature is constant, there is a linear relationship between time t and (1-α)ln(1-α)+α.