Provided are a method for preparing an oligomer and an apparatus for preparing an oligomer. The method for preparing an oligomer includes performing an oligomerization reaction by feeding a feed stream containing a monomer to a reactor; feeding a first discharge stream of the reactor to a first separation device, and feeding a second discharge stream of the reactor to a second separation device; feeding a lower discharge stream of the second separation device to a third separation device; feeding an upper discharge stream containing the monomer of the third separation device to a monomer dissolution device and dissolving the upper discharge stream in a solvent fed to the monomer dissolution device; and feeding a discharge stream of the monomer dissolution device to the reactor.