The invention discloses a method for preparing linear alpha-olefin. The method comprises the following steps of: introducing catalytic solution and ethylene into a first reactor and reacting at the temperature of between 30 and 120 DEG C and under the pressure of 0.1 to 3 MPa for 1 to 40 min to prepare a first mixed material flow of first linear alpha-olefin, the ethylene and the catalytic solution; introducing the first mixed material flow and the ethylene into a second reactor and reacting at the temperature of between 40 and 90 DEG C and under the pressure of 0.5 to 4 MPa for 5 to 120 min to prepare a second mixed material flow of second linear alpha-olefin, the ethylene and the catalytic solution; performing gas-liquid separation on the second mixed material flow to obtain the ethylene and a liquid-phase product; cycling the separated ethylene to the first reactor or the second reactor for continuous reaction; and performing distillation separation on the liquid-phase product to prepare the linear alpha-olefin. The method has the advantages of mild reaction condition, simple operation and suitability for industrialized production.