The purpose of this paper is to study the synthesis chemistry of supported Ziegler-Natta catalysts for polypropylene．Four kinds of catalysts were made，the first one (catalyst-A) from MgCl2-A solution containing MgCl2，EH(2-ethylhexanol) and PA (phthalic anhydride) dis-solved in decane；the second one (catalyst-B) from MgCl2-B solution containing MgCl2，EH and TBT (tetrabutoxy titanium)；the third one (catalyst-C) from MgCl2-C solution containing MgCl2，EH，PA and TBT；the fourth one (catalyst-D) from MgCl2-D solution containing MgCl2 and EH．The activity of catalysts varled in the order；B>C>A>D．Both activity and stereospecificity of catalyst-D ad'e very low．Catalyst-A and Catalyst-B have the same stru-ctures characterized bv X-ray diffraction．A mechanistic scheme for the variety of reactions that occur during the preparation of the final catalyst is proposed by analysis of IR spectra．The catalyst-B has about 60％ more activity and 2％ less stereospeci -ficity as catalyst-A．The pre-sence of BuO-group which has a less hindrance for monomer inserting has been identified as an improver of catalyst activity．The role of phthalic anhydride has been discussed in detail．Some“mono-esters”generated from the reaction between PA and ROH improved catalyst pro-perties．When PA was added during the preparation of catalysts，the finished catalyst produces polypropylene not only in high activity，but also in much better stereospecificity．