Disclosed in the present invention is a method for preparing nickel sulfate from a nickel-iron-copper alloy. The method comprises: in a high-pressure oxygen environment, mixing a nickel-iron-copper alloy crushed material, aqueous ammonia, ammonium sulphate, and a corrosion assisting agent, leaching, then performing solid-liquid separation on the leached slurry, adding a precipitant into a filtrate, and performing ammonia distillation to obtain a nickel-containing leachate; then adding an extractant into the nickel-containing leachate to extract nickel so as to obtain a nickel-containing extraction organic phase; and then adding sulfuric acid into the nickel-containing extraction organic phase to perform back extraction of nickel so as to obtain a nickel sulfate solution. According to the present invention, the nickel-iron-copper alloy is separated by using different properties of nickel and iron, nickel is dissolved in a hexamine complex of nickel, iron cannot be dissolved and then continues to be remained in a solid, after the filtrate is collected, the precipitant is added and ammonia distillation is performed to remove copper, the aqueous ammonia is recycled, and the copper ions react with the precipitant to generate a copper sulfide precipitate, and thus, copper in the filtrate is removed, and the purity of nickel sulfate is further improved.