Abstract: The integration of a large number of renewable energy power generation equipment has significantly changed the dynamic characteristics of modern power grid, which brings the challenges to the security and stability of the power grid. The subsynchronous/supersynchronous oscillation problem caused by the mismatch between the phase-locked loop bandwidth and the power grid strength is particularly serious. The previous studies have shown that grid-forming converters whose external characteristics are similar to the voltage sources can reduce the small-signal stability problems of renewable energy power systems dominated by PLL and provide strong voltage support capability for the system. However, the mechanism of effect of energy storage system based on grid-forming converters on the stability of the multi-renewable-energy-infeed system and the optimal siting and sizing method are still unclear. Therefore, this paper reveals the mechanism of the energy storage system based on the gridforming converters for improving the small-signal stability of the multi-renewable-energy-infeed system. Then, from the perspective of improving the system stability margin, the optimal siting and sizing method of energy storage systems based on gridforming converters is studied. Finally, the effectiveness of the proposed method is verified by the example.