A DC power transmission system of a voltage source converter using a pulse-interleaving auxiliary circuit is disclosed. The converter system comprises an IGBT converter for converting an AC power to a DC power or the DC power to the AC power; an open Y-Y transformer and a Y-.DELTA. transformer for stepping up or stepping down the AC power having a predetermined magnitude; a capacitor for dividing a DC voltage; and a DC Auxiliary circuit composed of a normal transformer and half-bridge for overlapping a pulse type input voltage to increase the number of pulses of an output waveform. In using a DC auxiliary circuit composed of normal transformer and 3-level half-bridge to increase the number of pulses of the output waveform by superposing the voltage in the form of the pulse, a normal transformer may be used instead of the tapped transformer to reduce the size thereof and to obtain an accurate transformer ratio, and a 3-level half-bridge may be used instead of the H-bridge to reduce the switching loss.