Automatic generation control (AGC) is an important function in the power system dispatching automation system. The importance of AGC control function is becoming more and more prominent in the unified frequency control and electricity spot market environment. A grid level and provincial level AGC joint closed-loop testing scheme based on real-time simulation is proposed in this paper. Then the communication interface between real-time simulation platform and heterogeneous system of grid-province AGC as well as the real-time closed testing technologies are researched and developed. The data interface methods such as parallel synchronization of multi-GTNET cards and "single terminal interface-data forwarding" are proposed to solve the problem of massive real-time interface data in the grid-province AGC joint closed-loop testing. Finally, the parallel synchronous high-speed communication ability and interface delay characteristics of multi-GTNET cards are verified through the interface test case.