The invention relates to a slag cooler for cooling slag of a circulating fluidized bed boiler of a power plant, mainly designed to solve the problems of poor cooling effect and easy burnout of the traditional slag cooler. The slag cooler comprises an inner cylinder wall, an outer cylinder wall, a cooling water interlayer, a slag guide sheet, a slag inlet, a slag outlet end, a track, a track supporting wheel, a large sprocket, a water inlet pump and a water pumping pump, wherein a partition wall is arranged between the inner cylinder wall and the outer cylinder wall, a small support upright column is respectively arranged between the partition wall and the inner cylinder wall as well as between the partition wall and the outer cylinder wall, an inner sealing head and an outer sealing head are also arranged between the partition wall and the inner cylinder wall as well as between the partition wall and the outer cylinder wall, a pipeline is arranged on the water inlet pump and is connected with an inner ring of a water separator, the inner ring is provided with a water outlet and is connected with an outer ring in a sliding and sealing mode, a water outlet pipe is arranged on the outer ring and is connected with the inner sealing head, a pipeline is arranged on the water pumping pump and is connected with an inner ring of a water collector, a water inlet is arranged on the innerring of the water collector, the external of the inner ring of the water collector is connected with the outer ring of the water collector in the sliding and sealing mode, and a water inlet pipe is arranged on the outer ring and is connected with the outer sealing head. The invention has the advantages of good cooling effect and long service life.