The invention relates to a fluidized bed reactor and a method for methanation of gas mixture containing H2 and CO. The fluidized bed reactor of the invention comprises a material gas inlet 1, a catalyst outlet 2, a gas-solid separator 5, a product gas outlet 6, a catalyst inlet 7 and a material gas lateral line inlet, wherein one or more gas distribution plate 3 is arranged in the fluidized bed reactor in order to divide the reactor into several fluidized bed layers to form several catalyst dense-phase zones; a cooling water spraying device 4 is arranged above each layer of the catalyst dense-phase zone in order to control temperature of the reactor. Because the circulating cooling water directly contacts high-temperature product gas, the reactor of the invention strengthens heat transfer, is easy to control reaction temperature, reduces heat loss during the heat transfer and improves heat utilization rate during the reaction. The reactor has the advantages of large production flux, big operation elasticity, high CO conversion rate, high selectivity of produced methane, less catalyst dosage, high utilization rate of heat quantity and so on, so that the reactor has good industrial application prospect.