The mechanical properties, hydration process and microstructure characteristics of the calcined hydrotalcite modified cementitious material at different fly ash contents were analyzed. Based on the results by the compressive strength test calcined hydrotalcite can greatly improve the compressive strength of cement-fly ash mortar. The phase composition of hardened cement paste was characterized by X-ray diffractionand and thermogravimetric analysis. The results show that calcined hydrotalcite can promote the hydration of cement-fly ash cementitious materials, thereby increasing the content of hydration products. It is incidiacted that calcined hydrotalcite can accelerate the dissolution of fly ash, and its nucleation effect is beneficial to the formation of hydration products.