The invention relates to an olefin polymerization device. The device is characterized by comprising a polymerization reactor, a gas-liquid separator and storage tanks, wherein the polymerization reactor is used for carrying out a homopolymerization and/or copolymerization reaction on olefins; the gas-liquid separator is connected with at least one outlet of the polymerization reactor and is used for receiving unreacted gas from the polymerization reactor and separating liquid, obtained after the gas is condensed, from remaining gas; the storage tanks are separately connected with the gas-liquid separator and at least one inlet of the polymerization reactor and are used for receiving condensate separated from the gas-liquid separator and carrying out storage according to the variety of comonomers contained in the condensate; and according to the need of a target product, the liquid is introduced into the polymerization reactor or not, or is introduced into the reactor at interval of a period of time, so as to achieve the switching between copolymerization and homopolymerization of the olefins in the polymerization reactor, and between copolymerization and copolymerization of the olefins in the polymerization reactor. The invention also provides a method for carrying out olefin polymerization by using the polymerization device.