A method and an apparatus for preparing an oligomer, the method including supplying a feed stream including a monomer to a reactor to perform an oligomerization reaction, recovering unreacted monomer, and dissolving the recovered unreacted monomer in a solvent in a monomer dissolution device and supplying a discharge stream from the monomer dissolution device to the reactor. The method and the apparatus provide reduced investment costs and improved economic feasibility as there is no need to use a refrigerant at a very low temperature or install a separate compressor to recover and reuse the unreacted monomer in an oligomer production process.