The invention discloses a catalyst composition for ethylene oligomerization. The catalyst composition comprises chlorinated-2-isobutyryl-1,10-phenanthroline amine iron (II) which is shown in a formula (I) as described in the specification and is used as a main catalyst, an aluminum-containing cocatalyst, water and an organic solvent. In the formula (I), R1 to R5 are independently selected from the group consisting of hydrogen, C1-C6 alkyl groups, halogen, C1-C6 alkoxy groups and a nitro group. With the oligomerization catalyst composition and method provided by the invention for ethylene oligomerization, technical biases are overcome, ethylene oligomerization process is optimized, high oligomerization activity is obtained, the application amount of the cocatalyst is small, rapid initiation and stable operation of ethylene oligomerization process are realized, good balance is reached between a catalytic effect and cost, good practicality is obtained, and cost for ethylene oligomerization is greatly decreased; in overall consideration of the catalytic effect and production cost, the catalyst has wide industrial prospects.