Using the ferric phosphate slag, a by-product produced in the production of ferric phosphate, the precursor material for the lithium battery positive electrode, this paper investigated a green recycling process for the high value-added product iron phosphate dihydrate (FePO₄·2 H₂O). The product was prepared by acid leachation-copitation process. The effects of the acid dissolution stage (acid concentration, temperature, liquid-solid ratio (L/s), stirring rate) and the co-precipitation stage (precipitation temperature, aging time, pH value) on the morphology and particle size of the product were mainly investigated. The optimal process conditions for leaching iron phosphate slag: sulfuric acid concentration is 6 M, dissolution temperature is 80℃, liquid-solid ratio is 6:1 and stirring rate is 100 r/min. The conditions for co-precipitation to produce FePO₄·2 H₂O particles are as follows: pH value of 2, temperature at 70℃ and aging time of 2 h. SEM and XRD tests indicated that it has good crystallinity and uniform particle distribution, meeting the requirements of the precursor for cathode materials used in lithium-ion batteries.