NOVELTY - A solution containing vanadic sulfate prepared by reacting vanadium trioxide, sulfuric acid and additive, and a solution containing vanadium sulfate prepared by reacting vanadium trioxide, sulfuric acid, reductant and additive are mixed and decompressed at 60 degrees C. High purity nitrogen is supplied to the mixture and evaporated to obtain cathode electrolyte. The additive is sulfate, phosphate, acetate, glycerin, ethanol, phosphorus heteropolyacid (or its salt) and/or silicon heteropolyacid (or its salt). The reductant is sulfo compound without impurity.    USE - Manufacture of cathode electrolyte for vanadium redox battery.    ADVANTAGE - The cathode electrolyte is manufactured economically by a simple process using easily available raw materials. The cathode electrolyte has high vanadium concentration, excellent stability and can be used efficiently at low temperature.    DETAILED DESCRIPTION - High purity vanadium trioxide is added into sulfuric acid (400-500 g) and nitrogen with purity of 99.999% (oxygen content of less than 0.001%) is supplied to the solution. The contents are heated to 80-150 degrees C. An additive is added to the heated solution and the undissolved substances are filtered to obtain solution (I) containing vanadic sulfate. High purity vanadium trioxide and reductant (2.8-3.2 mol) in the ratio of 1:1 are added to sulfuric acid (400-500 g). Nitrogen with purity of 99.999% (oxygen content of less than 0.001%) is supplied to the solution and the solution is heated to 80-150 degrees C. An additive is added to the solution and the undissolved substances are filtered to obtain solution (II) containing vanadium sulfate. The solutions (I, II) are mixed in predetermined ratio to obtain mixture containing vanadium sulfate, vanadic sulfate, water and additive. The mixture is maintained at 60 degrees C, decompressed and evaporated for 2-3 hours after supplying high purity nitrogen, to obtain cathode electrolyte. The additive is sulfate, phosphate, acetate, glycerin, ethanol, phosphorus heteropolyacid (or its salt) and/or silicon heteropolyacid (or its salt), and is added in an amount of 10-20%. The reductant is sulfo compound without impurity.