Abstract: The circuit time delay of modular multilevel converter based high voltage direct current（MMC-HVDC） transmission systems may cause high-frequency oscillation of power systems. This paper establishes an equivalent model of the time-delay link based on the Pade approximation, and develops a state space model of MMC-HVDC system by equivalently representing the timedelay based on the fourth-order Pade approximation. By comparing the dynamic characteristics of the model with those of the detailed electromagnetic transient model in PSCAD/EMTDC under the step condition and the recurrence of high-frequency oscillation, the correctness of the developed state space model and the validity of the fourth-order Pade approximation for the timedelay equivalent are verified. Then, the influence of the time constant of the time-delay on the system eigenvalues and the highfrequency oscillation characteristics is studied. The results show that, based on the eigenvalue analysis method, the critical delay time that causes the system high-frequency oscillation can be obtained by analyzing the state space model of MMC-HVDC system considering the delay link characteristics. Finally, based on participation factor analysis method, the main factors that cause the system high-frequency oscillation are studied. The results show that the system high-frequency oscillation is mainly caused by the coupling between the characteristics of AC system and MMC-HVDC system with the time-delay link.