Provided in the present invention are an olefin polymerization apparatus and an olefin polymerization process. The following operations and effects can be realized by the apparatus or method provided in the present invention: the gas material discharged from the polymerization reactor is condensed, after the gas-liquid separation, the resulting gas portion is recycled to the reactor to form a circulation loop; the condensate can be selectively introduced to the polymerization reactor, in order to achieve the free switch between the homopolymerization reaction and the copolymerization reaction or between different copolymerization reactions, and at the same time the condensate can absorb the heat generated by the reaction. By using the apparatus and method provided in the present invention, polyolefin products having excellent mechanical properties and processability can be prepared as needed.