With the widespread adoption of grid-forming energy storage systems, scenarios involving independent operation of power grids are becoming increasingly common. Due to discrepancies in voltage parameters between islanded systems and the main grid, synchronization control is required prior to grid connection to prevent protection malfunctions caused by inrush currents. This paper proposes a synchronization and grid-connection method for islanded grids based on grid-forming energy storage. By utilizing a single phase-locked loop (PLL) and active power closed-loop control, precise phase tracking of the islanded grid voltage is achieved. Active and reactive power commands are generated in real time by the energy storage coordination control for regulation. Taking the State Grid Hubei Xingang 50 MW/100 MWh energy storage power station as a case study, synchronization function retrofitting, algorithm deployment, and on-site commissioning are carried out. Successful synchronized grid connection of a 110 kV islanded system integrating source, grid, load, and storage is achieved. The results demonstrate that at the instant of grid connection, the phase angle difference is less than 1°, the frequency deviation is below 0.005 Hz, and the voltage deviation is less than 0.5%. The proposed approach offers advantages such as low retrofitting costs, high reliability, and broad applicability. It provides a practical reference for scenarios such as black start, fault recovery, and flexible microgrid deployment, demonstrating significant potential for wider deployment.