Abstract: In this paper, taking Lu Xi back-to-back project as example, an accurate impedance model of voltage source converter based high voltage direct current（VSC-HVDC） system was established for the mid-and high-frequency resonance problem, and its impedance characteristics were analyzed. Firstly, the small signal models of phase-locked loop（PLL）, power control loop, dq0 positive and inverse transform, positive and negative sequence current control loop were established by means of harmonic linearization to obtain the accurate sequence impedance model of VSC-HVDC. Then, the influence of each control link on the impedance was analyzed, and the application range of the simple impedance model was obtained. Subsequently, the effects of various measures on improving impedance characteristics were discussed, including adding voltage feedforward 2-order low-pass filter, reducing system delay time, optimizing system control parameters, and adding delay compensation. Finally, the correctness of theoretical analysis was validated by the impedance scan results based on the PSCAD off-line simulation model and the RT-LAB control hardware-in-the-loop test platform.