l-Glutamic acid is used as an additive for the positive electrolyte of all-vanadium redox flow battery (VRFB), and its effect on the thermal stability and electrochemical activity is investigated. It is found that the addition of l-glutamic can significantly alleviate the precipitation of V2O5 from positive electrolyte. The conservation rate of V(V) ion can be as high as 58% after 2M V(V) solution being kept in 40°C for 89h. Besides, l-glutamic can also improve the mass transport and electrochemical performance of anolyte. A high coulombic efficiency of over 95% and energy efficiency of 74% are obtained. XPS spectra illustrate that l-glutamic can react with the surface of carbon felt electrode and introduce more oxygen-containing and nitrogen-containing groups, which should be responsible for the improvement of electrochemical performance.