A fluidized bed reactor, a device for preparing low-carbon olefin, and a method for preparing low-carbon olefin are provided. The reaction zone of the fluidized bed reactor is sequentially, from bottom to top, provided with a raw material first distributor, a raw material second distributor and a catalyst distributor; the catalyst distributor is connected with the catalyst second feeding inlet; a dense-phase zone is formed between the raw material first distributor and the raw material second distributor, and the area where the catalyst distributor is located is formed as a catalyst distribution zone connected with the dense-phase zone; and at least one catalyst first feeding inlet(s) is provided on the side wall of reactor of the dense-phase zone. The distribution of the catalyst and the full contact of the catalyst with the raw materials can be achieved using the fluidized bed reactor and the device.