The reaction of maleic anhydride ring-opening was calculated by Gaussian03. The Density Function Theory (DFT) method were employed to study the geometries of maleic anhydride and ethanol on the base of B3LYP/6-31G in the paper. The transitional states(Ts1,Ts2) of maleic anhydride ring-opening reaction were found by TS method and were proved by IRC calculation. The results showed that from the reactant to product, the energy reduced about 129.25337kJ/mol,The computation results showed that the reaction was exothermic and matched up well with experiment. Nowadays the MAH multipolymers were studied extensively.Because of C=C double bond has double substitutions and the great Spatial steric hindrance, MAH was always regarded as representative that couldn’t carry out homopolymerization[1,2]. Much attention has been paid to MAH polymerization(Homopolymerization and Copolymerization[5-15])gradually Long and his colleagues published papers on MAH homopolymerization in 1963. The related reports have been increased since MAH multipolymer was found to possess anti-tumor and biological activity. According to the polymerization mechanism，epoxide ring-opening esterification were divided into three kinds: cationic polymerization, anionic polymerization and coordination polymerization.The saturated polyesters were prepared through maleic anhydride and alkene oxide reaction, generally using anionic polymerization initiated by hydroxyl compound such as alcohol, water and carboxylic acid. maleic anhydride ring-opening reaction initiated by alcohol(Eq. 1) was discussed in this paper.