Disclosed is a method for producing a sulfur fertilizer by diammonium phosphate device, comprising: (1) powdered sulfur is added into a preset molten sulfur device and heated and fused into liquid sulfur; (2) the liquid sulfur is added into an inlet of a pre-neutralizing tank pump, and enters a pre-neutralizing tank; (3) ammonia and phosphoric acid are added into the pre-neutralizing tank, and prepared into diammonium phosphate slurry through reaction; (4) sulfuric acid is added into the slurry of the pre-neutralizing tank, and reacted with ammonia to produce ammonium sulfate; (5) the prepared liquid sulfur, the diammonium phosphate slurry and the slurry containing the ammonium sulfate are fed into an inlet of a guniting pump together to form a mixed material, and part of slurry serves as returning charge; (6) the mixed material is thrown into a pelletizer by the guniting pump for guniting and palletizing so as to obtain a granular material; and (7) the granular material enters a drier and dried by hot-blast air, and then crushed and sieved to prepare the sulfur fertilizer. The method can use the existing diammonium phosphate device to prepare the fertilizer containing three crop nutrient elements of nitrogen, phosphorus and sulfur, and exploits a convenient and smooth technological line for sulfur fertilizer production. The invention is applicable to renovation of variety for phosphorus fertilizer enterprises.