The invention discloses a method of partially converting phosphorous plaster to prepare side product ammonium sulfate of aluminum sulfate, which has the main steps of converting phosphorous plaster, filtering and separating converted substances, neutralizing converted liquid, concentrating, crystallizing and drying aluminum sulfate solution, drying filtrated cake and bauxite, blending and homogenizing raw material, calcining and cooling grog, modifying, grinding and homogenizing grog, processing calcination tail gas and the like. The mixed material satisfying materials required by the production of aluminum sulfate cement is achieved by partially converting the phosphorous plaster which does not need to be preprocessed. No limestone is required to be additionally added, the application range of the phosphorous plaster with high ferric and aluminum impurity contents is widened, the procedures and the converting equipment are simplified, ammonia consumption and operation difficulty are reduced, the cost is saved, the discharge of secondary polluting substances is eliminate, the heat and the carbon dioxide carried out by the calcination tail gas are recovered and utilized, and the energy consumption can be effectively reduced to further save the cost. In addition, the method does not have high requirements on the impurity content of the phosphorous plaster and has strong suitability on the raw material.