More and more renewable energy is incorporated into the power system through power electronic equipment, resulting in reduced system strength and physical inertia. This paper proposes a grid-forming control strategy for modular multilevel converter based high voltage direct current transmission system (MMC-HVDC) with MMC internal energy control which is connected with a weak AC system. Firstly, based on the droop grid-forming control,, inertia link and PI control are added, so that MMC can automatically track the load fluctuation and adjust its own power output, and realize zero steady-state error regulation of AC voltage and frequency. Then, aiming at the problem of internal energy fluctuation of MMC controlled by grid-forming control, a MMC internal energy compensation controller is designed, which can effectively reduce fluctuation of DC voltage of submodule capacitance. Finally, based on the PSCAD/EMTDC electromagnetic transient simulation platform, a MMC-HVDC transmission system was established, and two operating conditions of AC grid load increase and AC grid load reduction are selected to verify the feasibility and effectiveness of the proposed control strategy.