The high SO3 content of circulating fluidized bed combustion(CFBC) ash as compared to other coal ashes has significant effect on the hydration and performance of CFBC ash cementitious system. Calcined anhydrite was used to control the SO3 content of the CFBC ash Portland cement clinker(30∶70, by mass) system, and the influence of SO3 on the setting time, compressive strength, linear expansion ratio, chemically combined water content and hydration products was investigated. The results show that the initial and final setting time of the system can meet the standard for ordinary Portland cement if the SO3 content of the system ranges from 2.5% to 3.5%, indicating that the anhydrite of CFBC ash plays a role in the control of setting time; however, the efficiency of anhydrite is obviously inferior to that of gypsum. Moreover, the chemically combined water content of the system increases with the SO3 content due to the exciting effect of SO3 on the combusted clay minerals of CFBC ash. If the SO3 content of the system is lower than 3.5%, the system strength will increase with the SO3 content; however, if the content exceeds 35%, the production of ettringite and the resulting expansion will inevitably lead to the reduction of the strength. It is suggested that it is necessary to control the SO3 content of CFBC ash Portland cement clinker system within the range of 2.5% to 3.5%, if no gypsum is added to the system.