Abstract: To improve the power quality of the integration point of hybrid cascaded high voltage direct current（HVDC）transmission system and reduce the project occupied area and cost, this paper proposes an active harmonic compensation method based on modular multilevel converter（MMC）. Firstly, the topology and harmonic characteristics of hybrid cascaded HVDC transmission system are introduced. The high-voltage valve adopts the line commutation converter（LCC）, and the low-voltage valve adopts multiple MMCs. Then, the harmonic equivalent circuit of the system and the active harmonic compensation control strategy are described. In the strategy, the harmonics of LCC will be compensated by MMCs. Furthermore, the influence of active harmonic compensation on the steady-state operation of MMC are analyzed. Finally, a simulation model of hybrid cascaded HVDC transmission system is built in PSCAD/EMTDC simulation software, which verifies the availability of the active harmonic compensation control strategy in the steady-state operation, power change and time delay conditions of the system.