Effect of poly acrylic acid (PAA) addition on the thermal stability and electrochemical performance of the V(V) positive electrolyte for all-vanadium redox flow battery (VRFB) were investigated in this paper by ex-situ stability tests, UV–Vis spectrometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Cyclic voltammetry (CV) and charge-discharge test. The results indicate that PAA additives can improve the thermal stability of V(V) electrolyte. A small amount of PAA additives can slightly improve electrochemical performance of the positive electrolyte and the energy efficiency of the VRFB at room temperature. Besides, the cell employing the positive electrolyte solution with 3% PAA shows good cycling performance at 50 ℃, the charge capacity retention of which is higher than that of the cell without PAA during the cycling.