The invention provides an online gas detection method used for an all vanadium flow battery liquid storage tank. The liquid storage tank is provided with an online detection system, and a gas in the tank is detected and analyzed when an inert gas is introduced to dispel air in the liquid storage tank to obtain component information of the gas in the liquid storage tank in order to accurately control the use amount of a protection gas; and the state of the gas in the liquid storage tank is detected in real time in the battery running process, and the inert gas is timely introduced when oxygen and other electrolyte ion-sensitive gas components are detected in order to protect and keep comprehensive valence balance of ions in the electrolyte. The gas in the all vanadium flow battery liquid storage tank is detected and analyzed in an online manner through adopting the online gas detection system, so the use amount of the protection gas is accurately controlled, and waste of the production gas is avoided, thereby the building cost of all vanadium flow battery energy storage power stations is reduced, the vanadium battery is well protected, the running efficiency of the battery is improved, the service life of the battery is prolonged, the running cost is reduced, and the running economy of the energy storage power stations is improved.