Provided is a method for producing HPMCAS capable of reducing the average particle size of HPMCAS particles to an intended range without a pulverization step. Specifically provided is a method for producing hypromellose acetate succinate, comprising an esterification step of reacting hypromellose with an esterification agent in the presence of a catalyst to obtain a reaction solution containing crude hypromellose acetate succinate, a precipitation step of mixing the reaction solution with water to precipitate the crude hypromellose acetate succinate, thereby obtaining a hypromellose acetate succinate suspension, a liquid removal step of removing a liquid from the hypromellose acetate succinate suspension with a centrifugal decanter to obtain liquid-removed hypromellose acetate succinate, and a drying step of drying the liquid-removed hypromellose acetate succinate.
