Disclosed is a method for preparing 1-octene in the presence of a catalyst system for ethylene oligomerization, which includes: premixing a part of ethylene gas as raw material with a solvent, mixing with the components a+b, c and d of the catalyst system, and then sending in a reactor; directly sending the rest of ethylene gas in the reactor; discharging the resultant liquid from the upper part of the reactor into an overflow channel; adding a catalysis stopping agent to the overflow channel; and then separating the resultant liquid. The advantages of the method are high selectivity of 1-octene and high catalytic activity.