The effects of impurity, temperature, concentration of vanadium and sulphuric acid on the stability of electrolyte in vanadium redox flow batteries are studied. It is found that the sediment at positive electrodes is V2O5·1.6H2O, and the sediment at negative electrodes is V2(SO4)3·10H2O. Although impurities influence the stability of vanadium electrolyte to some extent, the matching relationship between the vanadium and H2SO4 concentration is more important for improving the electrolyte stability. Precipitation of V(V) ions from solution can be prevented effectively by increasing the concentration of H2SO4. While, too concentrated H2SO4 would favor the precipitation of V(III) ions from negative electrolytes. To avoid the sensitivity of vanadium electrolyte to impurities and concern about both the stability of positive and negative electrolytes, the concentration of H2SO4 should be raised to a certain extent to confirm the stability of positive electrolyte. At the same time, a moderate vanadium concentration should be employed to assure the stability of negative electrolytes. From the viewpoint of increasing the stability, the preferred vanadium electrolyte composition is 1.5–1.6M vanadium in 4–5M H2SO4 medium at temperature above −10°C.