The grid-forming (GFM) inverter control technique nowadays is the research hotspot because of its ability to support weak grid, enhance grid strength, and improve system stability in renewable energy generation and micro-grid. However, recent researches have observed the instability of the GFM inverter when it closely connects to the outer grid. To truly solve this instability problem and not just by adjusting control parameters, in this paper, a new voltage control method is put forward, which could enable the GFM inverters to operate well in even extremely strong grid conditions. The strong-grid-friendly mechanism of the proposed method is revealed by its equivalent virtual impedance. Root loci of system poles and simulation waveforms are presented to validate the availability and effectiveness of the proposed method.