The invention relates to a method for preparing high-purity iron oxide and trisodium phosphate by a ferro-phosphorus alkalization process, which comprises the following steps of: mixing ferro-phosphorus slag with calcined soda and then roasting the mixture; performing water immersion on a roasted product, and using filter residues obtained after the water immersion to prepare the high-purity iron oxide; and using the filtrate obtained after the water immersion of the roasted product to prepare the trisodium phosphate. The process provided by the invention can fully utilize the iron and phosphorus elements in ferro-phosphorus at the same time, and has a certain value in energy-saving and emission-reduction; the comprehensive utilization ratio of the ferro-phosphorus is high, the conversion rate of the iron element can reach 91.3 to 93.2, the conversion rate of the phosphorus element can reach 89.8 to 91.1 percent, the product added value is high, the prepared high-purity iron oxide accords with a national standard HG/T 2574-94, the iron trioxide content can reach 98.8 to 99.4 percent; and compared with the existing common method for producing high-purity industrial iron oxide, the method for preparing the high-purity iron oxide and the trisodium phosphate by the ferro-phosphorus alkalization process greatly reduces the production cost, has a simple production process and non-demanding conditions, and is easy to implement.