The invention relates to a preparation method of vanadylsulfate, in particular to a preparation method of vanadylsulfate for electrolyte of an all vanadium ion flow battery. The preparation method comprises the following process flows of: taking V2O5 as a raw material, mixing excess reaction dose of V2O5 and sulfuric acid, adding the mixture into a first section of airtight pressure kettle, introducing SO2 into the kettle, controlling the oxidation-reduction potential of the solution to be 400mV-600mV to reduce most V<5+> into V<4+>; adding the filtrate obtained through the first section of reaction into a second section of airtight pressure kettle, introducing SO2 again, controlling the oxidation-reduction potential of the solution to be 260mV-400mV to convert all of the V<5+> into V<4+>; leading an obtained vanadylsulfate solution into an evaporation crystallizer, dehydrating and encapsulating to obtain blue vanadylsulfate crystal. The invention has the advantages of no introduced impurities in the process of preparing vanadylsulfate, high purity of the product, short process flow, simple operation and continuous production, and can be applied to the electrolyte of the all vanadium ion flow battery.