The invention relates to a system and a method for testing an energy storage system in grid-connection/grid-disconnection. The test system comprises a rectifying unit and an inverter unit, wherein anAC side of the rectifying unit is connected to the power grid through a LCL filter, a DC side is connected to a DC side of the inverter unit, and an AC side of the inverter unit is connected to the tested energy storage system; and wherein the inverter unit consists of three single-phase inverters, each of which comprises a transformer and a capacitor corresponding thereto, a AC side of the single-phase inverter is connected to a primary winding of the transformer, and a secondary winding of the transformer is connected in parallel with the capacitor and then is connected to the tested energystorage system. According to the technical scheme provided by the present invention, various linear loads and non-linear loads as well as pulse loads are artificially simulated, the grid-disconnectionload capacity of the energy storage system is tested, and the power grid adaptability of the energy storage system under the grid-connection condition is tested simultaneously.