In this work, the effects of Al-modification on Phillips catalysts were investigated by high-resolution Xray photoelectron spectroscopy (XPS) and the performance in ethylene polymerization reactions. Density functional theory (DFT) method was used to achieve a deeper mechanistic understanding. The decreased Cr 2p binding energies (BE), related with an increase in electron density of chromium by Al-modification, have been confirmed by the DFT calculation. The ethylene polymerization activity over the Al-modified Phillips catalyst was increased. The molecular weight (MW) of polymer was decreased with the expansion of molecular weight distribution (MWD) to low MW side, which was in agreement with the decreased energy barrier difference between the chain transfer and chain propagation in DFT calculation. The initiation step undergoing a metallacycle reaction pathway was found through a spin crossover from the quintet energy surface via a minimum energy crossing point (MECP) to the triplet energy surface. Modeling results indicated that the promotion effect on polymerization activity from Al-modification for Phillips catalyst was mainly originated from the chain initiation reaction with lowered energy barrier for Al-modified Cr active sites. Improved environmental stress-crack resistance (ESCR) of polymer derived from the Al-modified catalyst could be expected from the less short chain branches (SCBs) distribution in the low MW side. (C) 2015 Elsevier B.V. All rights reserved.