In order to decrease the passive filter rating, suppress the large resonant peak when the passive part is used alone for the series-in series hybrid active power filter (SHAPF), a novel series-in SHAPF topology is proposed in this paper. It is a combined system of a traditional SHAPF and a capacitor which is connected in series with inverter and resonant with the inductor of the output filter at the fundamental frequency. It can decrease the harmonic voltage on the inverter, increase harmonic impedance of the main circuit and avoid the affection of the line and passive filter variations. Simulation and experimental results from a 10 kVA prototype verify that the proposed SHAPF can promote the efficiency of the shunt passive filters, decrease the rating of the inverter and improve the system filtering performance and reliability.