The invention relates to a desulphurization synergism device and method for exhaust gas coming out of a circulating fluidized bed boiler. The efficiency of the traditional in-furnace desulphurization is low and can hardly reach 90%, the ratio of Ca to S is high, and the absorbent consumption is high. The desulphurization synergism device comprises a desulphurization synergism reactor (7) which is connected with a post-desulphurization bag type dust remover (8) and an absorbent auxiliary adding device (5). The post-desulphurization bag type dust remover is provided with a process water device (6) and connected with an inducted draft fan (1). The induced draft fan is connected with a chimney (2) and a desulphurization synergism reactor lower flue duct through a clean gas recirculation device (3). A desulphurization ash recirculation device (4) is arranged below the post-desulphurization bag type dust remover. A spray nozzle of the process water device is a high-pressure backflow type atomization spray nozzle. The desulphurization synergism device and method are used for the exhaust gas coming out of the circulating fluidized bed boiler.