The invention discloses an olefin polymerization method and device. The method comprises the steps of providing a fluidized bed reactor, and carrying out olefin polymerization in the fluidized bed reactor. According to the method, hydrogen is enriched by virtue of a membrane separation device, copolymerized monomers are separated and enriched by virtue of a condensing device and a gas-liquid separation device, the temperature gradient and the concentration gradient of hydrogen in the fluidized bed reactor are intensified under a second-stage separation effect of the membrane separation device and the gas-liquid separation device, and a polyolefin product with good forming machinability and physical performance can be produced.