The invention provides an SoC balance control method of a serial mixed energy storage three-port converter. The control method mainly comprises outer-ring SoC balance control and inner-ring voltage and current control, SoC balance of a storage battery is realized in the outer ring, inner-ring control is composed of voltage and current closed-loop adjustment, and power distribution of the mixed energy storage system and stability of the output voltage of the converter are realized on the basis of virtual impedance control. The provided control method is simple and effective, modularized designis easy to realize, and stable system voltage output is realized while SoC balance of mixed energy storage components is realized.