The present invention relates to a catalyst for preparing 1-hexene through trimerization of ethylene, wherein the catalyst comprises a P-containing compound a, a chromium compound b and an activator c, and the P-containing compound is a diphosphine ligand with a biphenyl or binaphthyl skeleton. According to the present invention, the catalyst is used for ethylene trimerization reactions, wherein the 1-hexene selectivity is close to 100%, the subsequent separation process is avoided, the consumption of aluminoxane is reduced, and the industrial application cost is reduced.