The invention discloses an ethylene trimerization and tetramerization catalyst composition which comprises a ligand compound shown in formula I, a transition metal compound, an aluminum-containing promoter and tert-butyl hydroperoxide, and in the formula, R is hydrogen, alkyl, cycloalkyl or aryl; R' is hydrogen, alkyl, alkoxy, hydroxyl or halogen. The invention further relates to application of the catalyst composition in ethylene trimerization and tetramerization reaction. The catalyst composition has the characteristics of being high in catalysis activity, high in selectivity and the like and has relatively good industrial application prospects and economic values.