The model of limestone instead of lime in converter in accordance with the open blowing stage and slag-splashing furnace-protection stage of steel-making were established. Under different particle size (4mm~27mm) and heat transfer conditions (N2 or converter slag), the high temperature decomposition experiment of large particle size limestone was carried out. The decomposition conversion α of limestone was obtained by experiments, and the kinetics of isothermal heating of limestone was studied by selecting the most suitable integral form of the most probable function by the "mode coordination method". The results show that the conversion of limestone decreases with the increase of particle size and increases with the increase of temperature, and the activation energy decreases with the increase of temperature and increases with the increase of particle size. The decomposition modes were in accordance with random nucleation and subsequent growth pattern(A-E). The most probable integral forms of limestone decomposition are G(α) = [-ln(1-α)]n, n=3/4 (blowing) and G(α) = [-ln(1-α)]n, n=1/2 (slag splashing), respectively.