In this paper, the structure and principle of a kind of co-phase power supply system is proposed. The co-phase power supply devices based on two-level quadruple modular topology structure and three-level duplicate topology structure are studied and the former one is chosen as a computational example. According to the analysis of high power electronic elements in power system, a k/n(G) reliability model of the co-phase power supply device, based on two-level quadruple modular topology structure, is established. Then, the reliability of the co-phase power supply device is calculated. With the failure rate and Mean Time To Failures (MTTF) presented, the reliability of the co-phase power supply device is quantificationally evaluated. In order to promote the reliability of the device, two optimized schemes, which include increasing internal components of converter cells and increasing the standby branches of the device, are presented. Compared with the original scheme, the optimized schemes show the promotion of the reliability. Finally, to demonstrate, the computational method is adopted. The results agree well with the proposed schemes.