In ethylene polymerization catalyzed by α-diimine nickel complex, the introduction of 1,2-diketopyracene backbone could greatly improve thermal stability, and the bulky benzhydryl-substituted aniline has the same effect. Herein, two strategies above were combined to obtain α-diimine nickel complexes with different numbers of benzhydryl substitutions at the ortho- and para-positions of aniline. In ethylene polymerization, all complexes exhibited high polymerization activity and enhanced thermal stability. Among them, Ni3 had the best thermal stability. Furthermore, in the analysis of thermal stability, the analytical methods of computational chemistry were used and multiple influencing factors such as steric hindrance effect, electronic effect, complex stability and non-covalent interaction were combined. It is believed that the π-π stacking interaction can greatly improve the catalytical activity of Ni3 at elevated temperature probably due to the inhibition of chain transfer reactions. Thus the thermal stability of α-diimine nickel complex, expressed by the appearance polymerization activity, is determined by the combined effect of multiple influencing factors.