The national standard “Technical Specification for Grid-Forming Converters in Electrochemical Energy Storage Systems” (draft for comments) requires grid-forming energy storage system (GFM-ESS) to achieve long-duration low-voltage ride-through and transient synchronization stability under faults. To quantify this capability, a parameter feasible region construction method is proposed for maintaining voltage-source characteristics and synchronization stability under GFM-ESS sustained faults. Firstly, a fault-current engineering calculation method considering electromagnetic transient process is developed to obtain the transient current peak and steady-state AC component as current constraints. Then, an energy function incorporating virtual impedance and equivalent damping coefficient is constructed as the synchronization stability constraint, while the feasible active-power reference range is used as the equilibrium-point constraint. On this basis, an expression is derived for the parameter set that enables the GFM-ESS to maintain voltage-source transient synchronization stability under faults. The feasible region is further characterized using the active-power reference and virtual reactance as key parameters, thereby quantitatively determining the stable operating boundary of the GFM-ESS. Finally, the effects of various grid-side and device-side parameters on the feasible region boundaries, together with their underlying mechanisms, are quantitatively analyzed, and electromagnetic transient simulations are conducted to verify the effectiveness of the constructed feasible region.