Abstract: A unified modeling method for AC-side and DC-side impedance of modular multilevel converter（MMC） is presented in this paper. Dynamic phasors were used for electrical modeling of MMC, which is based on the rotating d-q frame and considers the circulating current. The 10 th-order dynamic phasor model of MMC was linearized and transformed into the s-plane by Laplace transform and combined with the control system. The frequency-domain model representing the closed-loop control dynamics of MMC was obtained. Then, the AC/DC-side impedance model of MMC was derived based on transfer function matrix. The comparison with electromagnetic transient simulation shows that the AC/DC-side impedance models derived in this paper have satisfactory accuracy within 1～3000 Hz. Finally, the AC-side impedance was used to analyze a MMC high-frequency resonance accident, and the DC-side impedance is used to analyze the influence of the smoothing reactor on the stability of point-to-point HVDC. The analysis results further validate the accuracy of the proposed model. The high-frequency oscillation will occur when the delay is high, and large smoothing reactor will lead to DC resonance of flexible HVDC transmission.