In a two-stage inverter, the downstream inverter with single-phase or three-phase unbalanced load will generate an instantaneous power fluctuation of the second harmonic frequency. This will be coupled into the front-end DC converter and result in a large second harmonic fluctuation of the AC resonant current amplitude in the front-end dual-active-bridge series resonant converter (DBSRC). Taking into account the complexity of the high-order model of DBSRC, the simplified model of a reduced first-order small signal model is proposed. Based on the simplified model, the output impedance at the second harmonic is greatly increased by inserting a double frequency notch in the voltage closed-loop, so that the resonant current fluctuation of the front-end DBSRC can be suppressed. Further, a parameter design method of the control system to suppress the second harmonic fluctuation is proposed, and the control strategy to ensure the stability of the two-stage inverter is realized. Finally, the feasibility and effectiveness of the proposed simplified reduced-order model and the design approach of control parameters are verified by simulation and experiment.