A catalyst for synthesizing 1-hexene from ethylene trimerization and its application are provided. Said catalyst consists of (a) the compound containing P and N, (b) electron donor, (c) Cr compound, (d) carrier and (e) accelerator. The molar ratio of (a), (b), (c), (d) and (e) is 0.5-100 : 0.5-100 : 1 : 0.5-10 : 50-5000. The catalyst is prepared by mixing the components of (a)-(e) in an ethylene trimerization apparatus in situ and ethylene is introduced into the apparatus continuously. The prepared catalyst can be used to synthesize 1-hexene from ethylene trimerization in the inert solvents. The trimerization is performed at 30-150°C and 0.5-10.0 MPa for 0.1-4 hours. The catalyst has high catalytic activity and high 1-hexene selectivity. During the process of ethylene trimerization, by-product polyethylene does not stick to the apparatus.