The invention discloses a measuring method for state of charge of series-connected batteries, which aims to solve the inaccurate problem of the SOC measuring method in the prior art. According to the SOC measuring method, firstly, the maximum available capacity of each single cell in the series-connected batteries, the residual capacity or the SOC of the single cell is measured, and then, the maximum rechargeable capacity, the maximum discharging capacity or the residual capacity of the series-connected batteries under the same state is determined to be respectively equal to the maximum rechargeable capacity and the maximum discharging capacity in the minimum in the single cells, the maximum available capacity of the series-connected batteries is equal to the sum between the maximum rechargeable capacity and the maximum discharging capacity of the series-connected batteries by calculation, and finally, the state of charge SOC of the series-connected batteries is calculated to be equal to the residual capacity divided by the maximum available capacity of the series-connected batteries. By using the SOC measuring method, the SOC of the series-connected batteries can be accurately obtained, and the reasonable basis is provided for the grouping, the application, the maintenance and the replacement of the series-connected batteries.