Calcium aluminate special cement with rapid early-age strength development and high-temperature resistance is frequently utilized for well cementing operations in heavy oil thermal recovery. It is essential for understanding the effect of early-age hydrothermal curing on the structural stability of cement binder to maintain cementing safety and oil recovery effectiveness. The strength development, mineral composition, and microstructure of cement cured at 20, 50 and 80 ℃ were investigated. The results show that the main mineral phases formed in calcium aluminate cement, such as gehlenite, CAH10 and C2AH8, gradually transform into granular hydrogarnet with poor cementation during hydrothermal curing, resulting in loose and porous structure of cement binder, which leads to the decline in compressive strength. The incorporation fly ash and slag can not effectively prevent the crystal transformation and structural damage. The modification of sodium hexametaphosphate maintains the structural stability of mineral phases, and the dissolved Na+ and HPO-4 ions released from hexametaphosphate react with Ca2+ and ［AlO4］5- ions released from cement to generate sodium calcium aluminophosphate hydrate (N-C-A-P-H) gel phase products, which further enhances the density and cementation performance of cement binder.