Ammonium sulfate double salt crystals (ASDSCs) are formed during the electrolytic production of manganese. Typically, the large volume of ASDSCs accumulates in the open air, and this leads to serious environmental pollution and wastage of resources. In this study, we developed a new double-membrane three-chamber electrolysis method. In this method, ASDSCs were dissolved in water and then pretreated stepwise to precipitate manganese(II) carbonate and magnesium carbonate. These precipitates were filtered and the filtrate (mainly ammonium sulfate) was subjected to double-membrane three-chamber electrodecomposition to produce sulfuric acid and ammonia. Further investigations showed that under the optimal conditions of current density of 250 A/m2, electrolysis time of 18 h, and temperature of 40 °C, the decomposition rate of ammonium sulfate reached as high as 96.15%. Thus, using this method, ASDSCs can be completely decomposed, which resolves the problem of environmental pollution and provides certain economic benefits to enterprises.