Abstract: With the increasing scale of new energy installed, the demand of power grid for energy storage is also greatly increased in the future to realize the flexible adjustment of power grid operation. The increase of grid connected battery energy storage system（BESS） capacity has a significant impact on the operation characteristics of power system, especially on the small signal stability. In this paper, firstly, the mathematical model of BESS was analyzed to establish the power system model with large capacity BESS. Taking the three-generator and nine-bus system as an example, the eigenvalue analysis method was used to study the influence of different grid connected capacity of BESS on the small signal stability of the system when the power flow is changed or unchanged. The results show that the influence of the increase of BESS permeability on the small signal stability of the system is obviously different when the power flow is changed or unchanged. On the whole, BESS participating in the coordinated control of system operation mode is more conducive to the small signal stability of the system. In addition,eigenvalue analysis verifies that the virtual inertia control can damp the system oscillation to a certain extent and improve the small signal stability of the system.