The invention discloses a fluid bed polymerizaing method and the reactor. Polymerized monomer, copolymerized monomer, molecular-weight adjusting gas, nitrogen gas, condensate, catalyzer as well as aid-catalyzer enter in the reactor to make polymerization, part of gas stream from the top of the reactor circulates by compressor and is cooled to liquefacient temperature by heat exchanger, the obtained two streams enter in A and B areas of the reactor; the two streams return to the reactor in gas-liquid mode, and the liquid is separated out and directly leaded into the fluid bed; the polymerization: temperature 70-130 deg.C, pressure 1-10 MPa.