Through the reaction heat release of calcined clay in alkaline solution， the influence rule and mechanism concerning calcium hydroxide content，reaction temperature， calcination temperature and kaolinite content on the reactivity of calcined clay were investigated. Results show that the reactivity of calcined clay can be easily characterized by the reaction heat release analysis， and there is a good linear correlation between the cumulative heat release and the kaolinite content of the clay. With the increase of calcination temperature， the clay particles are broken， the degree of structural disorder increases， and the reaction begins and gradually accelerates to its peak，then the clay particle size further increases and the reactivity decreases significantly.