TiCl 4 /MgCl 2 catalysts are still the main industrial catalyst for isotactic polypropylene at present. However, the mechanism of isotactic polymerization of propylene has not been fully understood. DFT calculations revealed that the bare Ti active site was regioselective and nonstereoselective in the absence of an electron donor. The role of electron donor ethyl benzoate (EB), diethyl-2,3-diisobutylsuccinate (DiBS), cyclohexylmethyldimethoxysilane (CMDMS), and dicyclopentyldimethoxysilane (DCPDMS) on the active site was investigated. The presence of EB, DiBS, CMDMS, or DCPDMS around the Ti active site can promote the activity and retain the regioselectivity. It is worth noting that in the presence of EB and DCPDMS, the stereoselective behavior can be promoted with the advantage of 1 kcal/mol. The copresence of AlEt 2 Cl species and external donors can increase both the stereoselectivity and regioselectivity.