Abstract: Severe conditions in regenerators of FCC processes are harmful to catalyst structure, activity, selectivity and life. Hydrothermal stability study is one of effective methods to learn the deteriorating mechanism of the catalysts. By studying the hydrothermal stability of a FCC catalyst, it is found that the total surface acid sites decrease sharply between 750 and 800C . The ratio of strong acid to total acid sites increases with the decrease of framework aluminum. At 850C when the steaming time reaches 4h, the unit cell size shrinks to 2.427nm and Si/Al ratio rises to 20.1. Most acid sites are strong in this situation and furthermore the relative damage degree of the zeolite is more than 60%.