Two-stage single-phase inverter system has an inherent low-frequency power problem. In this paper, a power decoupling method based on differential split-capacitor design is proposed for IPOS-CLLLC single-phase inverter system, which can transfer all second harmonics ripple power to the DC bus capacitor without additional power decoupling circuit. By accurately controlling the charging and discharging process of the capacitors, the proposed method not only significantly suppresses the second-order oscillation of bus voltage and input current, but also effectively reduces the bus capacitance requirement, providing feasibility for high power density design in power supplies. Theoretical analysis and simulation verification are provided in this paper.