A parallel-connected twelve-pulse thyristor converter consisting of two slx-pulse groups with an interphase reactor has been used for low-voltage high-current dc power supplies because of its high efficiency compared with a series one. It seems to be the most suitable converter for superconducting magnetic energy storage systems (SMES), which are expected to be used in actual power systems in the near future as a power system stabilizer. Harmonic reduction on the source side is required for such large-capacity thyristor converters. A number of methods have been proposed. These methods, however, require additional devices . Thus, a simple parallel-connected converter without any additional devices remains to be constructed to reduce harmonic currents on the source side. This paper attempts to construct a simple 12-pulse thyristor converter by proposing a novel design concept of the interphase reactor. This method proposed here requires no additional devices, and only reducing the inductance of the interphase reactor. The source current of the 12-pulse thyristor converter with the reduced inductance interphase reactor approaches sinusoidal waveforms, which are approximately equivalent to those of the 36-pulse thyristor converter. Simulation results show that decreasing the inductance of the interphase reactor contributes to reducing harmonic currents on the source side. In addition to the simulation results, experimental results verify the validity and practicability of the proposed novel design concept of the interphase reactor.