Abstract: In order to study the characteristics of the heterogeneous traffic flow mixed with connected automated vehicles and human-driven vehicles, the stability and fundamental diagram model of the mixed traffic flow of connected automated vehicles and human-driven vehicles is proposed. Firstly, four different driving modes in the mixed traffic flow composed of connected automated vehicles and human-driven vehicles are analyzed. Then, based on the Intelligent Driver Model with different parameters, the mixed traffic flow stability discriminant and fundamental diagram model are derived. Finally, a microscopic simulation environment is constructed based on SUMO simulation software. The results show that as the penetration rate of connected automated vehicles increases, the critical speed range for the stability of the mixed traffic flow gradually decreases. At the same stable speed, the increase in the proportion of connected automated vehicles is conducive to the increase in traffic density and volume. All these prove that connected automated vehicles are not only conducive to the stability of mixed traffic flow, but can also improve the traffic efficiency of the transportation system.