This paper proposes a half-bridge inverter based Active Power Quality Compensator (APQC) using a constant DC capacitor voltage control for electrified railways. The APQC is composed of three-leg structured power devices with two common DC capacitors. Two legs perform as two half-bridge inverters, which compensate the unbalanced active, reactive and harmonic currents on the source side through DC capacitor. The third-leg controls two common DC capacitors voltages with a small amount of the output currents. Therefore the current rating of the third-leg devices can be reduced as compared to that of the already proposed three-leg structured APQC. For the control algorithm of the half-bridge inverter based APQC, a constant DC capacitor control based algorithm is used. Any calculation blocks of the unbalanced active, reactive and harmonic components of the load current are not needed. Thus we offer the simplest algorithm for the half-bridge inverter based APQC. The basic principle of the proposed half-bridge inverter based APQC using a constant DC capacitor voltage control is discussed in detail, and then confirmed by digital computer simulation using PSIM software. A prototype experimental model is constructed and tested. Experimental results demonstrate that the proposed APQC can achieve balanced and sinusoidal source currents on the primary side of Scott transformer in the traction substations for electrified railways.