With the increasing penetration of renewable energy, the demand for reactive power compensation capability and inertia control in new power systems has significantly risen. The grid-forming reactive power static var generator (SVG) based on supercapacitor can provide short-term inertia support to the power system through the supercapacitors on the DC-link and grid-forming control, while retaining the reactive power compensation capability of the SVG itself, thereby enhancing system stability. This paper presents the topology and control strategy of a grid-forming SVG based on supercapacitor, establishes an equivalent model of the supercapacitor, analyzes its charging and discharging characteristics, and designs the key parameters of the energy storage system with inertia support capability in the context of a 35 kV/50 Mvar grid-forming SVG system. Finally, the obtained parameters are validated on the PLECS, confirming their effectiveness.