Grid-forming (GFM) control is an effective way for power electronic converters to enhance the stability of a power system dominated by renewable power sources. In high-voltage and high-power application scenarios, GFM converters usually adopt modular multilevel converter (MMC) topology. Because of the absence of a centralized filter capacitor on the MMC AC side and the difficulty in obtaining current feedforward, the performance of AC voltage-AC current cascade control loops in conventional GFM control methods may deteriorate. To address this, a novel AC voltage control strategy is proposed. While retaining the current limiting capability, the current inner loop is transformed into a controllable voltage source in series with a virtual impedance using the Thevenin-Norton equivalent, thereby achieving a terminal voltage construction mechanism similar to that of synchronous generators. Moreover, the control loops and parameters are optimized using small signal state space modelling. Finally, simulation results in PSCAD/EMTDC prove the effectiveness of the proposed strategy and parameter design method.