Abstract: Activating the vast potentials of demand response（DR）in loads is currently an important research topic of the smart grid.However,because of the characteristics of mass,diversity and responsiveness of DR resources,it is a huge challenge to incorporating the DR resources in real-time simulation.A digital real-time co-simulation method of the smart grid for integrating large-scale fast DR resources is proposed.First,the network with a large number of DR resources is decoupled from the rest of the grid,which asks for building the multi-rate digital simulation according to different dynamic responses.Then,an interface algorithm including loop control is designed for the co-simulation.After that,typical application scenarios about microgrid and high-voltage grid are analyzed,and constitutions of the digital real-time co-simulation system are given based on RTDS and GridLAB-D.Finally,by taking the large-scale air-conditioning load as an example,the effectiveness of the proposed co-simulation method is verified.By using personal computers,the method is able to extend the simulation ability of existing digital real-time simulation systems for large-scale fast DR resources,which shows the good scalability and economic benefits of the proposed method.