The invention discloses a method for preparing simvastatin ammonium salts from lovastatin. The method comprises the following steps: firstly, carrying out ring opening and side chain hydrolysis on lovastatin to obtain red aspergin J acid; secondly, providing an acyl part of an acyl donor for C8 hydroxyl of the red aspergin J acid in the presence of acyltransferase; thirdly, adjusting the acidity of a simvastatin reacting solution, and separating out a simvastatin acid solid; finally, salinizing simvastatin solid ammonium salts to obtain the simvastatin ammonium salts. According to the method disclosed by the invention, reaction steps can be reduced, operation time is shortened, an experiment process is simplified, pollution to the environment is reduced, and the quality of a product is improved.