The invention provides a fluidized bed reactor which comprises a first reaction zone, a second reaction zone and a transition zone, wherein the first reaction zone is arranged above a distributor; the second reaction zone with an expanding diameter is arranged above the first reaction zone; and the transition zone is constructed between the first transition zone and the second transition zone. The invention also provides an olefin polymerization device and an olefin polymerization method based on the fluidized bed reactor. On one hand, the invention can effectively guarantee the stable fluidization of the fluidized bed reactor and can greatly increase the production load of the reactor, and on the other hand, the product property can be improved and the branching degree of the product can be increased when the olefin polymerization device and method are used for producing polyolefin.