Grignard reagent PhMgCl was added during the preparation of the catalyst system MgCl2/di-n-butyl phthalate (DNBP)/TiCl4-AlEt3/diphenyl dimethoxyl silane (DPDMS) to improve its performance. It was found that PhMgCl could enhance both the activity of the catalyst and the isotacticity of the products, but decreased the Ti content of the catalysts under the same preparation conditions. The polymerization kinetics showed that PhMgCl accelerates the decay in the same time that it increased the initial and final polymerization rates. By means of UV-vis spectroscopy, electron spin resonance (ESR) spectroscopy, and the Ti content determination of the catalysts, the multiple roles of PhMgCl were disclosed : reduction of Ti4+ to Ti3+, association with MgCl2 to replace part of TiCl4 in aspecific active sites, and complexing with the original active sites to form new active sites.