The invention relates to a synthesizing method for low-cost battery-grade iron phosphate. The synthesizing method comprises the following steps of purifying, wherein a ferrous sulfate aqueous solution is prepared, a purifying agent is added, filtering is carried out, and ferrous sulfate filter liquor is obtained; mixing, wherein a phosphorus source aqueous solution is prepared, the phosphorus source aqueous solution and the ferrous sulfate filter liquor are stirred and mixed, and a mixture solution is obtained; oxidizing, wherein hydrogen peroxide is dropwise added into the mixture solution, Fe<2+> is oxidized into Fe<3+>, and Fe3+ is obtained through precipitation; separating, wherein a filter cake is washed until eluate is neutral and contains no SO4<2-> ions; drying, wherein the washed filter cake is fully dried, then grinding and screening are carried out, and the battery-grade iron phosphate (FePO4.2H2O) powder is obtained; evaporating, wherein the filer liquor and ammonium hydroxide are subjected to neutralization, then evaporating and crystallizing are carried out, and ammonium sulfate fertilizer is obtained. The method is low in cost, simple in process and easy to control, and obtained iron phosphate is small in granularity and high in purity; besides, it can be considered to recycle water obtained through evaporation as reaction liquor, and thus economic benefits are created while the environment is protected.