A method and an apparatus for preparing an alpha-olefin. The method includes supplying a feed stream including a gaseous ethylene monomer to a monomer dissolution device to dissolve the feed stream in a solvent and form a liquid ethylene monomer, and supplying a feed stream including the liquid ethylene monomer as a discharge stream to a reactor, thereby removing heat of dissolution of the gaseous ethylene monomer outside of the reactor, and decreasing an amount of a refrigerant used in an alpha-olefin production process to improve economic feasibility.