The grid-forming (GFM) energy storage-based static synchronous compensator (ES-STATCOM) operates as a voltage source and offers virtual inertia and damping. These capabilities make it more suitable for the modern power system with high renewable energy penetration than grid-following (GFL) static synchronous compensator (STATCOM). However, it is prone to overcurrent during grid faults. Physically increasing the overload capacity of the device is not a cost-effective solution. In this paper, a current limiting strategy based on virtual sequence impedance for the GFM ES-STATCOM under asymmetrical faults is proposed. It enables the GFM ES-STATCOM to maintain its GFM control mode during transients. The control for activation and deactivation of current limiting is first designed. Then, the transient virtual sequence impedance control is developed to limit the fault current amplitude, along with the instantaneous overcurrent peak suppression method. In addition, key control parameters are designed through analysis of fault current characteristics. Finally, the proposed strategy is validated by PSCAD/EMTDC simulation under asymmetrical faults.