The utility model discloses a half water reaction system of phosphoric acid, including dissolving tank and crystallizer tank, dissolving tank and crystallizer tank series connection, be provided with in the dissolving tank and dissolve agitating unit, be provided with crystallization agitating unit in the crystallizer tank, dissolving tank bottom sets up the discharge gate and passes through the dissolving tank pump and carry the ground paste to the crystallizer tank in, and still be provided with the back flow between dissolving tank and the crystallizer tank, the crystallizer tank top sets up sulphuric acid pan feeding mouth, and still be connected with vacuum apparatus on the crystallizer tank, the crystallizer tank obtains the result and carries to subsequent filtration system, thoughtlessly groove is being connected in advance to the pan feeding mouth of dissolving tank, it sets up backward flow diluted acid pipeline on the groove to mix in advance, the dilute sulfuric acid pours into into to mix in advance through backward flow diluted acid pipeline to be carried after the inslot mixes with the phosphorus ore to the dissolving tank in again. The utility model discloses effectively solve plaster of paris crystalline solid little and unstable, sadly strain, problem that the scale deposit is serious, control half water calcium sulfate crystallization speed, crystal growth easily filters with higher speed, improves the yield of phosphoric acid.