Abstract: In the context of metro permanent magnet traction system applications, the current control loop is prone to instability. Firstly, the current loop closed-loop transfer function of PMSM based on complex vector current controller is derived, and its decoupling performance under variable speed condition is discussed. Secondly, the influences of time delay and angle delay on the stability of the control system are analyzed according to the zero-pole diagram of the current-loop transfer function, and the stability boundary conditions are derived considering the digital control delay comprehensively. Then, the mechanical motion equation of permanent magnet synchronous motor is linearized, the closed-loop transfer function of the system is derived, the zero-pole diagram is drawn, and the stability boundary conditions under different working conditions are analyzed. Finally, experiments verify the correctness of the theoretical analysis.