The invention discloses an olefin polymerization method and device. The method comprises the following steps: introducing a liquid material I and a catalyst I into a first reactor, and enabling olefin in the liquid material I to be in contact with the catalyst I to generate polymerization reaction to form polyolefine; leading the liquid material I and polyolefine out from the discharging opening of the first reactor to be introduced into a second reactor, meanwhile introducing a catalyst II, a gas material and a liquid material II into the second reactor; arranging a dense-phase fluidized bed inside the second reactor, and enabling the gas material, the liquid material II and the catalyst II to be in contact to generate polyolefine; guiding out the unreacted gas material from the outlet of the second reactor, conducting compression, condensation and gas-liquid separation, conveying the liquid material II obtained through gas-liquid separation to the first reactor to form a circulation loop I, and conveying the residual liquid material II and gas material to the second reactor to form a circulation loop II; continuously or intermittently discharging polyolefine from the second reactor. The invention provides a reactor system for the method.