Abstract: The limestone decomposed to activity lime by TG at the rates of 10, 15, 20, 25 and 30℃/min was studied. The decomposing mechanism equation is F(α)=(1-α)^-1, and the reaction rate constant is $k=54325{{\rm{e}}^{-\frac{{98873}}{{RT}}}}$. It is founded that the diffused constant in the decomposition of lime from limestone is $\lg D=-\frac{{2506}}{T}+2.28$, a dynamics model of decomposing time of limestone with the decomposition ratio X_B was revised, and the finished time of limestone decomposition at 1423 K was forecasted with this model. It indicates that the forecasted results agree very well with the experiment results.