<p>Modular multilevel converter (MMC)-based high voltage direct current (HVDC) transmission with grid-forming control (GFM) is suitable to integrate weak AC grids with high share of renewable energy. To solve the instability problem of GFM in strong grid, the unified GFM is proposed for stable operation of MMC-HVDC under different short circuit ratio (SCR). In addition, the unified GFM is able to control DC voltage, AC power and capacitor voltage for frequency support. Then, based on the network-based stability analysis, the hierarchical stability analysis including AC side stability and DC side stability is presented to evaluate the DC network stability for asynchronous AC network interconnection. The AC side stability is ensured by the active damping control for a wide range of SCR. With the AC side stability of each MMC, the DC side stability can be ensured using the generalized Nyquist criterion based on the DC impedance/admittance. Furthermore, to simplify the DC side stability analysis, the passivity of the DC impedance/admittance is ensured with the proposed co-design of GFM control, active damping and DC side virtual impedance, so that the stability is easily ensured as long as the AC side stability of each MMC is ensured. Comparisons between our unified GFM control/stability analysis and the conventional control/stability analysis are presented. Simulations validate our analysis.</p>