Effects of operation parameters including bed temperature, Ca/S molar ratio, excess air coefficient, fly ash recirculation rate and limestone microstructure on SO2 emission were investigated on a 410t/h circulating fluidized bed (CFB) boiler burning petroleum coke and coal. Results show that for different fuels, SO2 emission is correspondingly related to the sulfur content in it under the same operation conditions. With increasing bed temperature, SO2 concentration in the flue gas reduces first and then increases. There is an optimal desulfurization temperature. For burning bituminous coal (BC) only or 70% BC + 30% petroleum coke (PC), the optimal desulfurization temperature is about 850 °C, while it is about 850–870 °C for burning 50% anthracite (AN) + 50% PC. SO2 emission decreases with the increase in Ca/S ratio, excess air coefficient and fly ash recirculation rate. Microstructure of limestone has distinct effects on their SO2 retention capacity, and bigger specific surface area and higher specific pore volume lead to stronger SO2 capture activities. The optimal temperature, Ca/S ratio and excess air coefficient for different fuels are recommended for industrial application.