Through the reaction heat release of calcined clay in alkaline solution, this study investigated the influence law and the influence mechanism of calcium hydroxide content, reaction temperature, calcination temperature and kaolinite content on the reactivity of calcined clay. Results showed that the reactivity of calcined clay can be quickly characterized by the reaction heat release analysis, and there was a good linear correlation between the cumulative heat release and the kaolinite content of the clay. With the increase of calcination temperature, firstly, the clay particles were broken, the degree of structural disorder increased, and the reactivity appeared and gradually increased to the peak value, then the clay particle size further increased and the reactivity decreased significantly.