Abstract: With large scale offshore wind farms connecting to the grid by MMC-HVDC system, the onshore and offshore AC power grids are decoupled, which leads to the continuous decrease of the total effective inertia and increases the pressure of the stable operation of the power system. The European offshore wind power project bidding and related standards have clearly proposed that the MMC-HVDC system should provide the function of inertia support for the AC power system. For this problem, this paper proposes two strategies to improve the inertia time constant of AC power system: one using the capacitances of MMCs modulars, and other configurating energy storage device based on the virtual synchronous generator strategy. According to the definition of capacitance time constants, the control characteristics and parameters of MMC-HVDC, the DC voltage control strategy based on inertia support is derived, and the influence of parameters of HVDC system on inertia time constant is analysed. Based on the design consulting project of MMC-HVDC for IJmuiden Ver ± 525 kV/2 GW offshore wind farms integration of TenneT grid company of the Netherlands, the simulation and validation of two inertia support strategies in time domain is conducted. The influence of changing inertia time constant on the design scheme is given, and the two strategies are compared and illustrated.