The use of MMC converters virtually decouples offshore wind farms from the AC grid, which shall modify system inertia to affect its capability to ride through the disturbances of system frequency. In this paper, the existing inertia controllers for MMC-HVDC based offshore wind farm integration are classified into two categories and their working principles and control strategies are summarized. On this basis, a synthetic inertia controller was proposed. It can be regarded as a combination of two mainstream strategies and has the advantages of both but avoids their respective shortcomings, which could provide larger and longer synthetic inertia support. Finally, the dynamic performance of the MMC-HVDC system has been successfully demonstrated by PSCAD/EMTDC simulations for the proposed synthetic inertia controller. The simulation figures demonstrate that the proposed controller is capable of delivering the desired active power response to a rate of change of frequency (RoCoF) step change in the onshore network.