Power hardware-in-the-loop (PHIL) simulation which combines the advantages of physical experiments and real-time digital simulation is a promising method for future complex large-scale power electronic system research. The power interface realizes the effective interconnection of physical and digital systems, but it is also a main factor that affects the stability of the PHIL system. This paper presents a DC PHIL system using the ideal transformer model algorithm and establishes a mathematical model of the power interface. The influence of the power interface on the system stability is analyzed by Routh criterion and Bode plot method. Simulation and experimental results are used to validate the correctness of the theoretical analysis and the effects of the power interface.