An apparatus for preparing polyalpha-olefins includes a mixing unit, a microchannel reaction unit, a high-pressure separation unit, a low-pressure separation unit, a gas circulation unit, a post-treatment unit and a pressure control unit, the mixing unit, the microchannel reaction unit, the high-pressure separation unit, the low-pressure separation unit that are successively connected. The gas circulation unit, the microchannel reaction unit is provided with the BF3 gas inlet, the mixing unit is provided with the auxiliary feed inlet, and the olefin raw material inlet, the gas circulation unit is connected with the BF3 gas inlet, the low-pressure separation unit is further connected with the post-treatment unit, and the high-pressure separation unit, the pressure control unit, and the gas circulation unit are further successively connected. The apparatus has the advantages of high polymerization reaction speed, high reaction conversion and good product selectivity, and is suitable for large-scale industrial production.