Abstract: Hydroxypropyl methylcellulose acetate succinate (HPMCAS) was successfully synthesized from hydroxypropyl methylcellulose (HPMC), which was used succinic anhydride and acetic anhydride as esterifying agent, acetic acid as organic solvent. By varying the amount of factors, HPMCAS with the different content of succinyl groups and acetyl groups were prepared. The contents were determined by titration, and then the structure of the product was characterized and tested. The results show that the reaction system was heated at 105oC for 6.5 h, after reaction we use deionized water for the precipitation agent, make products precipitated in 750 rpm speed. We can obtain that the succinyl groups and acetyl groups were about 13% and 8% respectively. The obtained HPMCAS products have the polydispersity index of 1.06 and the molecular weight distribution is narrow. The product has a good solubility in acetone, methanol and other solvents and the thermal stability is very good before 200 oC.