The invention discloses a circulating fluidized bed boiler with second reheaters. The boiler comprises an air compartment, an air distributor, a combustion chamber, a primary air system, a second air system, a horizontal flue, a plurality of cyclone separators, a plurality of external heat exchangers and a back-end flue. The contraction ratio of the lower part of the combustion chamber is 0.2-0.49. A flue gas damper is arranged in the back-end flue. The back-end flue contains at least two parallel flues. A low-temperature superheater is arranged in one parallel flue. A primary low-temperature reheater is arranged in the other parallel flue. A medium-temperature superheater is arranged in at least one external heat exchanger. A primary high-temperature reheater is arranged in at least one external heat exchanger. A second low-temperature reheater and a second high-temperature reheater are arranged in at least one external heat exchanger. A high-temperature superheater is arranged at the upper part of the combustion chamber. The boiler can effectively raise the heat-transfer intensity inside the boiler, and cooperates with optimized heating surface arrangement modes to achieve effective combination of second reheaters and a circulating fluidized bed, and therefore the aims of raising power generating efficiency of coal power units and reducing pollutant discharge are achieved, and long and high-efficient running of the device can be guaranteed.