The effects of chloro compounds on the catalytic performance in ethylene trimerization reactions were studied using a homogeneous [Cr(2-ethylhexanoate)(3)/2,5-dimethylpyrrole (DMP)/triethylaluminum (TEA)/chloro compound] catalyst system. Chloro compounds of different types showed a broad range of effects on the performance of the catalytic system. The chloro compounds of geminal chloro groups greatly improved the catalytic activity and I-hexene selectivity of ethylene trimerization reaction. Some specific interaction modes of chloro groups with dimeric TEA and active Cr species in the catalytic cycle were proposed to explain the catalytic activity and I-hexene selectivity improvement effects of chloro compound in the ethylene trimerization reaction.