Provided are a method for preparing an oligomer and an apparatus for preparing the same. The method for preparing an oligomer including: supplying a feed stream including a monomer to a reactor to perform an oligomerization reaction; supplying a first discharge stream from the reactor to a first separation device and supplying a second discharge stream from the reactor to a second separation device; recovering the monomer as an upper discharge stream from the second separation device and supplying a lower discharge stream from the second separation device to a third separation device; and supplying an upper discharge stream from the third separation device to the second separation device.