Poor power quality and phase splitting are the main issues in electric railway. Co-phase traction power supply system is adopted in electrified railway for active power balance, reactive power compensation and harmonic filter. The research development and application of co-phase traction power supply system in China are depicted detailed in this paper. Experiments and pilot project proved that good power quality, feasible, reliable and economical traction power system can be implemented in co-phase traction power supply system.