The invention relates to a fluidized bed reactor, which comprises a reaction chamber, a solid substance separator, a device which can send back the separated solid substance to the reaction chamber. The separator has a separating chamber, and the separating chamber is provided with a curved upper wall (9) and a lower wall (10), thus forming an air outlet (13). The upper wall and the lower wall are connected with a solid substance returning pipe (11). The lower wall is preferably made into a plurality of oblique surfaces or oblique slots (35) directing to the solid substance returning pipe (31). |