In order to reduce the switching frequency of railway power conditioner in the process of high power compensation, a hybrid cascade 81-level converter with low switching frequency is used as the voltage source and a single-phase H bridge converter with high switching frequency is used as the current source. The two voltage sources are connected back-to-back, and an 81-level railway power conditioner is proposed. Since the switching frequency of the switching devices that bear 85% of the compensation power in the system is 250 Hz and below, and the current sources with high switching frequency only bear 1% of the power, the system has small switching loss and good tracking performance for the compensation current reference signal. The basic structure of the system and its equivalent circuit is studied, the equivalent three-phase circuit is established by constructing the equivalent symmetrical three-phase voltage source, the active power average distribution method is used to make the system equivalent to Y-Y symmetrical three-phase circuit, so that the imbalance compensation is realized, and a new compensation current detection method is put forward. Simulation results show the effectiveness and feasibility of the proposed system and its compensation method.