At present, the power grid is carrying out the construction of unified frequency modulation market, and the difficulty of automatic generation control (AGC) function test in the actual power grid will be further increased. Dispatching automation urgently needs a simulation environment that can simulate the actual power grid to support the AGC function test. Real time simulation has the ability to carry out AGC closed-loop test in terms of technical feasibility. In order to verify its effectiveness, this paper builds a AGC closed-loop test system based on real-time simulation, and inverses the actual AGC action events in the field. The test results show that the RTDS real-time simulation model has the ability to simulate the frequency change of large power grid system, and the RTDS system has the ability to apply the closed-loop test with AGC, which can help the AGC function test of power grid.