Method for preparing oligomer

A method for preparing an oligomer, the method comprising:

• supplying a monomer stream comprising a monomer and a solvent stream comprising a solvent to a reactor to perform an oligomerization reaction to prepare a reaction product;
• supplying a discharge stream from the reactor to a separation device, wherein the discharge stream comprises the reaction product, and supplying a lower discharge stream from the separation device to a settling tank;
• adding an organic flocculant to the settling tank to settle and remove a polymer from the lower discharge stream, wherein in the settling tank, the lower discharge stream from which the polymer is removed is separated into an upper layer liquid including a solvent and a lower layer liquid including the oligomer, and
• supplying an upper layer liquid stream comprising the solvent to a solvent separation column and a lower layer liquid stream comprising the oligomer to a high boiling point separation column;
• and
• removing a high boiling point material from a lower portion in the high boiling point separation column and supplying an upper discharge stream including the oligomer to the solvent separation column.