Fe2(SO4)3 was investigated as an additive to improve the thermal stability of the positive electrolyte for an all-vanadium flow battery (VFB). The effects of the Fe2(SO4)3 addition on the electrochemical activity, capacity, and charge/discharge cycle performance were also studied. The addition of Fe2(SO4)3 significantly enhanced the thermal stability of V(V) in the positive electrolyte. For a positive electrolyte consisting of 2.0 M vanadium ions, the addition of Fe2(SO4)3 within the concentration range of 0.5–0.7 M enabled the electrolyte to remain stable for more than 168 h at 50 °C. Cyclic voltammetry (CV) measurements showed that the peak positions of the Fe2+/Fe3+ and VO2+/VO2 + couples did not cross each other, indicating that the Fe2(SO4)3 addition did not lead to any side reactions. Electrochemical impedance spectroscopy (EIS) analysis revealed that the addition of Fe2(SO4)3 led to a slight increase of the electrolyte ohmic resistance and reduction of the charge-transfer resistance on the carbon electrode. Both the CV and EIS results indicate that Fe2(SO4)3 had no adverse effects on the reversibility or electrochemical activity of VO2+/VO2 +. The capacity for battery charge/discharge cycles hardly changed compared with that of the pristine electrolyte during operation for over 360 h at 50 °C. The improved vanadium positive electrolyte with the Fe2(SO4)3 addition is expected to be used for practical applications. The Fe2(SO4)3 addition can extend the VFB operation temperature to 50 °C, thus reducing the cost of thermal management of the energy storage system and improving the technical and economic performance of VFBs.