Power quality conditioners based on modern power electronics technology were proposed to solve the power quality problems of the electrified railway power supply system. Large-capacity power converters are used as the main circuits of the compensator, which is one of the main reasons for the high initial cost of the railway power conditioner. A hybrid power quality conditioner (HPQC) for co-phase power supply system in electrified railway is proposed in this study. The HPQC adopts a single-phase back-to-back converter. It connects to the feeding phase of the balance feeding transformer via an L-C branch and to the other phase via a coupling transformer. To inject the same compensating currents to the traction power supply system, the DC bus voltage of the HPQC could be much lower than that of an active power conditioner (APC). As a result, the cost of the power quality conditioner is reduced. Simulation models are built with a HPQC connected to the secondary side of a 110 kV/27.5 kV V/V transformer. Simulation results show the HPQC could compensate reactive current, unbalance current and current harmonics simultaneously. Comparisons with an APC are also given. A small-capacity experimental prototype is built in the laboratory to validate the HPQC and testing results are also provided.