The invention provides a gas-phase polymerization reaction device which comprises a main reactor, a reactor cover, a gas inlet pipeline, a gas exhaust pipeline and a stirrer. The top of the main reactor is in sealing connection with the reactor cover, the stirrer comprises a stirrer shaft and a stirring paddle fixedly or movably connected with the stirrer shaft and located in a cavity of the main reactor, the stirrer shaft penetrates through the reactor cover in sealing mode and extends out of the main reactor, and the gas inlet pipeline and the gas exhaust pipeline penetrate through the reactor cover in sealing mode and are communicated with the cavity of the main reactor. The invention further provides an olefin polymerization method which is achieved by using the gas-phase polymerization reaction device. The gas-phase polymerization reaction device is used for performing olefin polymerization, can prevent unreacted monomer or hydrogen from being accumulated in the cavity of the main reactor in a polymerization process, and guarantees that composition of gas mixtures in the cavity of the main reactor remains unchanged basically in the polymerization process.