In this study, unlike conventional methods for producing vanadium (3.5+) electrolyte by VOSO4 and V2O5, a batch-type hydrothermal reactor was used to produce a vanadium (3.5+) electrolyte for vanadium redox flow batteries through a reduction reaction. The starting material, V2O5, was mixed with different concentrations of citric acid (0.8M, 1.2M, 1.6M, 2.0M) as reducing agent and stirred using a hot plate at 90°C for 60 minutes to achieve complete dispersion in the solution. The resulting solution was then subjected to a hydrothermal reduction reaction in a furnace at 150°C for 24 hours to produce vanadium (3.5+). The mixed state of the produced vanadium (3+) and vanadium (4+) was confirmed using UV-vis spectroscopy. Electrochemical properties were investigated through CV analysis, confirming that the optimal concentration was 1.6M. Charge and discharge experiments were conducted to compare the current efficiency, energy efficiency, and voltage efficiency of the electrolyte prepared by the hydrothermal reduction process and the electrolyte prepared by VOSO4. As a result, the electrolyte produced through the hydrothermal reduction process showed improved performance in all efficiencies. The results shows that vanadium (3.5+) electrolyte could be easily produced through a reaction process using citric acid.