The reaction activities for a series of benzylidenequinolin nickel complex systems are studied by density functional theory methods. The effective net charge values obtained on nickel atoms illustrate the correlation with experimental activities. Catalytic activity increases with the higher effective net charge values. The energy gaps (Δε 1) between Ni complex's lowest unoccupied molecular orbita (LUMO) and ethylene's highest occupied molecular orbital (HOMO) orbitals are calculated, regarding both pre‐catalysts and active species. The results show that there are also relationships between Δε 1 and catalytic activities. Comparing the pre‐catalysts to the active species, the energy gaps Δε 1 decrease dramatically from about 60 kcal mol–1 to about 0 kcal mol–1 for each complex, indicating the important activating role played by the co‐catalyst. Finally, it is found that the energy difference values between different spin states relates to catalytic activities for both pre‐catalyst and active species as well. Ethylene Rreactivity catalytic activities for a series of nickel complex catalysts are evaluated by density functional theory methods. The calculations indicated that, besides the correlation with effective net charges on nickel atom, the energy gaps between the complex and ethylene, and the energy difference values between the two spin states, also present relationship with activity.