Highly active ethylene polymerization catalysts based on a new four-component catalyst-system Ti/Mg/THF/improver (Cl3CCH2OH) and with SYLOPOL948 or SMR49-4049 silica as carrier were prepared and studied. Compared with the corresponding catalysts based on a three-component catalyst system (Ti/Mg/THF), these new catalysts were improved in catalytic performances such as catalytic activity and hydrogen response. In particular, the catalysts based on the four-component catalyst system and SMR49-4049 silica had very high catalytic activity. The effects of improver Cl3CCH2OH were investigated. The investigations into the catalyst formation mechanism showed that after dehydrated silica was treated with AlEt3, the average pore diameter of silica was enlarged from 20.3 to 26.2 nm for SMR49-4049 silica and from 25.3 to 34.9 nm for SYLOPOL948 silica. This transformation favors the impregnation of catalyst components on silica. After the catalyst was prereduced with alkyl-aluminum, the pore diameter of the final catalyst was enlarged from 17.5 to 26.4 nm for the catalyst based on SMR49-4049 silica, and from 27.6 to 45.2 nm for the catalyst based on SYLOPOL948 silica. This transformation favors high catalytic activity.