The invention provides an olefin polymerization device, which comprises a reactor, a recycle gas heat exchanger, a gas-liquid separation tank and a device for removing powder and entrainment, wherein the device for removing powder and entrainment is positioned between the reactor and the gas-liquid separation tank, and comprises a first opening and a second opening; the first opening is communicated with the recycle gas heat exchanger; the second opening is directly communicated with the reactor or is communicated with the liquid phase outlet of the gas-liquid separation tank; the first opening is used for discharging a gas phase; and the second opening is used for discharging a liquid phase and a solid phase. The invention also provides a method for polymerizing olefins by using the olefin polymerization device. In the olefin polymerization device, the device for removing powder and entrainment is arranged between the reactor and the gas-liquid separation tank, so that powder and entrainment in recycle gas of the reactor can be captured; therefore, the operating stability of the olefin polymerization device is improved.