To study the characteristics of the mixed traffic flow composed of manually driven vehicles and connected automated vehicles in the intelligent connected environment, mainly exploring the relationship between flow, density, and speed, this paper constructs a basic graphical model of the mixed traffic flow in the intelligent connected environment. First, four different car-following models in the mixed traffic flow are analyzed, as well as the corresponding percentage of following modes under different penetration rates of connected automated vehicles. Based on this, the different following models, and the corresponding model parameters, are applied to derive the fundamental diagram model of mixed traffic flow in the intelligent connected environment. Finally, the simulation is verified by designing SUMO simulation experiments. The simulation results show that the flow-density data scatters obtained under different connected automated vehicle occupancy ratios are in high agreement with the theoretical curves, which verifies the feasibility of the model.