Abstract: Modular multilevel converter (MMC) has outstanding advantages in the application of new energy transmission, island power supply, and etc, adopted in more and more applications. Due to the large number of sub-modules in MMC, the electromagnetic transient simulation is difficult and time-consuming, which increases the difficulty of design and optimization of MMC and the stability analysis of power system with MMC. Therefore, an accurate and efficient simulation model should be established to analyze the system in different operation conditions. In this paper, based on the operation-mode of the MMC sub-module, a working mode of the MMC bridge arm was analyzed. And then, an equivalent aggregated model was derived for system-level analysis. Finally, a 49-level MMC detailed electromagnetic transient model and its equivalent aggregation model were built in PSCAD/EMTDC. The dynamic processes of the two models in the startup, operation and fault of VSC-HVDC were compared. The results show that the proposed MMC aggregation model can accurately simulate system-level behavior of MMC under various working conditions.