Co-phase power supply technology has gained significant attention in research circles as it offers an effective solution to the issue of neutral sections in traditional railway power supply systems. This paper explores the stability behaviors of the co-phase compensation device-network-locomotive system. The study is divided into three main segments: modeling, stability assessment, and experimental validation. Initially, a thorough mathematical model of the entire system is developed. Subsequently, stability analysis is performed utilizing eigenvalue analysis to study how the co-phase compensation device affect system interactions. Finally, experiments using Starsim hardware-in-the-loop are conducted to confirm the findings of the stability analysis. The experimental results indicate that the incorporation of the co-phase compensation device can reduce the oscillation amplitude by nearly 20%. However, when the number of locomotives connected to the traction network in the depot increases to 7, the interaction between the locomotives and the co-phase compensation device introduces a new oscillation mode, reducing the stability margin. These findings provide a theoretical basis for the design of oscillation suppression and co-phase compensation device.