The invention discloses a method and device for deeply purifying a by-product ammonium sulfate solution from diachylon ammonium desulphurization. The method comprises the steps of primary purificationof the ammonium sulfate solution, adjustment of the pH value, evaporative crystallization, deep purification and oxidization. The device comprises a desulphurization by-product storage tank, a quantitative ammonia water adding device, a primary purification reactor, a primary purification filter, a quantitative sulfuric acid adding device, a pH value adjusting reactor, an evaporation crystallizer, a centrifugal machine, a crystallization mother liquor storage tank, a quantitative ammonium sulfide adding device, a quantitative polyacrylamide adding device, a deep purification reactor, a deep purification filter, a quantitative hydrogen peroxide adding device, an oxidization reactor, a pipeline filter and a delivery pump. By means of the method, high-purity ammonium sulfate which meets therequirements for the superior product in GB / T 535-1995 is prepared, and efficient recycling of resources is achieved. According to the method, the ammonium sulfate is recycled from a filtrate obtainedafter purification of diachylon desulfurization mother liquor, and the method has the advantages of being low in energy consumption, high in product purity, economical and environmentally friendly. The situation that waste liquid containing heavy metals is discharged into the environment to cause environmental pollution is avoided.