Abstract: To optimize electrolyte ratio of the vanadium redox flow battery and set a reasonable charge and discharge system, the free vanadium ion activity, ion-pair dissociation constant and other thermodynamic properties should be clarified. The conductivity of the VOSO4·3.53H2O(s) in aqueous solution was determined using the electric conductivity method at 298.15 K. The limiting molar conductance was obtained using the Origin data fitting and then the activity coefficient was solved with the improved Ostwald dilution law and the Davies equation, so as to obtain the true solution ionic strength. The vanadyl sulfate ion-pair dissociation constant was calculated through the Fuoss method. The vanadyl sulfate limiting molar conductance Λ0 is 203.252 032 5 S·dm²·mol^-1 and the vanadyl sulfate ion-pair dissociation constant Kd is 0.001 961 62, which may provide a basic data for the battery thermodynamic properties.