Large current discharge to battery, which is required in order to measure the ohmic internal resistance for stateof charge(SOC) estimation adopted by conventional internal resistance method, may pose security risks to electrical load during online operation. To solve this problem, this paper proposes a technique based on harmonic impedance measurement technology to estimate storage battery SOC online: discharges the objective storage batteries with periodic and small current, measuring the discharge current and terminal voltage of the battery during the discharge process, calculate the harmonic impedance of the storage battery according to Fourier analysis technique, least squares method is used for numerical simulation of harmonic impedance data to get the objective battery ohmic resistance, and finally realizes the online estimation for the storage battery SOC without the large current discharge operation required in conventional methods.