The invention relates to a mixed energy storage system comprising an accumulator group (10), a control loop (20) of a static switch, a super capacitor group (30) and a load (40). During charging, the super capacitor group (30) is firstly charged; when voltage at both ends of the super capacitor group (30) is raised to a set value, the control loop (20) of the static switch is controlled, so that the static switch is conducted; during discharging, the super capacitor group (30) is firstly discharged; when the voltage at both ends of the super capacitor group (30) is lowered to the set value, the control loop (20) of the static switch is controlled, so that the static switch is conducted; and the accumulator group (10) charges the super capacitor group (30) and also supplies power to the load (40). The invention realizes the stabilization of the voltage at a load end by controlling the static switch, meets the requirement of load power, also reduces the charge/discharge frequency of accumulators and achieves the aims of decreasing the configuration capacity of the accumulators and prolonging the service life.