A method for determining an actual charging capacity of a flow battery, to monitor the actual charging capacity of a flow battery which comprising a stack, an anode electrolyte storage tank, a cathode electrolyte storage tank and an electrolyte circulating pipeline, the method comprising the following steps: Step 1: determining a state of charge, "SOCs", of at least two pairs of different monitoring positions, any pair of monitoring positions being as followings: an inside of the anode electrolyte storage tank and an inside of the cathode electrolyte storage tank, inside an anode electrolyte outlet pipeline of the stack and inside a cathode electrolyte outlet pipeline of the stack, or inside an anode electrolyte inlet pipeline of the stack and inside a cathode electrolyte inlet pipeline of the stack; Step 2: configuring coefficients according to a ratio of a power to a capacity of the flow battery system; according to the SOCs respectively corresponding to each pair of monitoring positions, acquiring a SOC sum of a flow battery system by calculating the SOC sum of the flow battery system according to a formula SOCsum = ∑ Nn X SOCn, wherein Nn are coefficients, N is the sum of the Nn coefficients and N is equal to 1, n corresponds to pairs of monitoring positions, n is equal to 2 or 3; and taking SOC sum as a calculated SOC of the flow battery; Step 3: obtaining current operating state parameters of the flow battery; Step 4: determining the actual charging capacity of the flow battery according to the calculated SOC of the flow battery and the obtained current operating state parameters of the flow battery and in combination with the corresponding relationship among the actual charging capacity of the flow capacity, and SOC of the flow battery and operating state parameters of the flow battery. Step 1: determining a state of charge, "SOCs", of at least two pairs of different monitoring positions, any pair of monitoring positions being as followings: an inside of the anode electrolyte storage tank and an inside of the cathode electrolyte storage tank, inside an anode electrolyte outlet pipeline of the stack and inside a cathode electrolyte outlet pipeline of the stack, or inside an anode electrolyte inlet pipeline of the stack and inside a cathode electrolyte inlet pipeline of the stack; Step 2: configuring coefficients according to a ratio of a power to a capacity of the flow battery system; according to the SOCs respectively corresponding to each pair of monitoring positions, acquiring a SOC sum of a flow battery system by calculating the SOC sum of the flow battery system according to a formula SOCsum = ∑ Nn X SOCn, wherein Nn are coefficients, N is the sum of the Nn coefficients and N is equal to 1, n corresponds to pairs of monitoring positions, n is equal to 2 or 3; and taking SOC sum as a calculated SOC of the flow battery; Step 3: obtaining current operating state parameters of the flow battery; Step 4: determining the actual charging capacity of the flow battery according to the calculated SOC of the flow battery and the obtained current operating state parameters of the flow battery and in combination with the corresponding relationship among the actual charging capacity of the flow capacity, and SOC of the flow battery and operating state parameters of the flow battery.