In order to improve the energy density and battery efficiency of vanadium redox flow battery based on sulfate-chloride mixed acid electrolyte, the viscosity, conductivity and electrochemical performance of electrolytes with different concentration compositions are investigated systematically. The viscosity of electrolytes with four valences both increase with vanadium concentration, sulfate concentration and chloride ion concentration, the conductivity increase with sulfate concentration and chloride ion concentration but decrease with vanadium concentration. The results of electrochemical measurements indicate that the electrolyte with 2.2 M vanadium concentration, 2.75 M sulfate concentration and 5.8 M chloride ion concentration presents the best electrochemical performance due to common effect of the viscosity, conductivity and active vanadium ion concentration. The battery performance of electrolytes show that the ohmic polarization, concentration polarization and electrochemical polarization of electrolytes have great influences on the charging and discharging voltage platform which leads to the change of battery efficiency. In consideration of the improvement of battery efficiency and capacity, the electrolyte containing 2.2 M vanadium concentration, 2.75 M sulfate concentration and 5.8 M chloride ion concentration is selected as the optimum combination which can provide theoretical direction for the engineering application of vanadium redox flow battery based on sulfate-chloride mixed acid electrolyte.