A process for preparing 1-hexene under existance of ethylene oligomerizing catalyst system includes such steps as mixing solvent with the components in said catalyst system, including chromic compound, pyrrole derivative, alkyl aluminium and improver, loading in reactor, mixing ethylene gas with cyclic reactive liquid, spraying it in reactor, discharging the resulatant liquid, cooling part of the resultant liquid and mixing it with said ethylene, and loading the rest of the resulant liquid in separator. Its advantages are high activity of catalyst and high transform rate of ethylene.