The invention provides a SOC online estimation method based on an all-vanadium flow battery mathematical model and relates to the technical field of battery management. The method mainly comprises thesteps of (1) obtaining estimation parameters and measuring terminal voltage U0, terminal current I0 and pump loss Ipump of three groups of all-vanadium flow batteries in an operation process to obtain three sets of estimation parameters, (2) introducing the parameters obtained in step (1) into a mathematical expression of the all-vanadium flow battery equivalent circuit model E-U0p- (Ipump+I0)R1=U0 and calculating open circuit voltage E, equivalent internal resistance R1 and external parasitic loss equivalent resistance R2 of an all-vanadium flow battery, and (3) introducing data obtained instep (2) into the follow formula E=N*[1.219+0.11841g(1-1/SOC)], and calculating and obtaining an SOC value according to the formula. According to the method, the separate adding of one single batteryto detect the open circuit voltage of the all-vanadium flow battery is not needed, the open circuit voltage is calculated by using the all-vanadium flow battery equivalent circuit model, the cost of an all-vanadium flow battery system is saved, and the implementation is easy.