The invention discloses a circulating fluidized bed boiler capable of adjusting a circulating ash flow. The circulating fluidized bed boiler comprises a hearth, an ash return pipe, a tail flue, a cyclone separator and an ash returner, and is characterized by further comprising an ash conditioner and a slag cooler, wherein the ash conditioner consists of a venturi fluidized air chamber and a cylindrical ash cooling chamber; the venturi fluidized air chamber is communicated with the ash cooling chamber; a first flow equalizing annular plate, a snakelike cooling coil and a second flow equalizing annular plate are connected into the ash cooling chamber; the first flow equalizing annular plate and the second flow equalizing annular plate are both porous structures; the venturi fluidized air chamber is formed by an expansion section, a cylinder section and a shrinkage section, which are communicated with each other; the ash cooling chamber is connected with an ash return valve of the ash returner through an ash discharge pipe; an ash return pipe is connected between the ash return pipe and the ash cooling chamber; an isolating valve is mounted on the ash return pipe; and the venturi fluidized air chamber is connected with the slag cooler through the ash discharge pipe. The circulating fluidized bed boiler is advantageous in that, circulating ash can be introduced to the boiler to adjust the load inside in a large range, so that the combustion efficiency of a fuel in the boiler is greatly improved, and the inner wall wear of the boiler is reduced.