A moderate temperature dry circulating fluidized bed flue gas desulfurization (CFB-FGD) process was developed using rapidly hydrated sorbent. This technique has the advantages of low cost, no water consumption, and a valuable dry product CaSO4. To keep the system operation stable, a mass balance model, based on cell model considering flow state, particle abrasion, particle residence time, particle segregation and desulfurization processes, was built to predict the system state and optimize the operating condition. Experimental studies were conducted on a pilotscale CFB-FGD system with rapidly hydrated sorbent made from CFB circulating ash and lime (circulating ash sorbent) or coal fly ash and lime (coal fly ash sorbent). Calculated results were compared with experimental results and the relative error was less than 10%. The results indicated that feed sorbent mass, feed sorbent size, superficial gas velocity, particle abrasion coefficient and cyclone efficiency had significant influence on the mass balance of CFB system. The circulating ash sorbent was better than the coal fly ash sorbent, for providing higher desulfurization efficiency and being better for the CFB-FGD system to achieve mass balance.