This study investigates the effect of connected automated vehicles (CAVs) platoon characteristics on the fundamental diagram under mixed traffic flow. First, a new platoon intensity of CAVs is developed to measure the clustering strength of CAVs. Second, a new traffic flow model is proposed based on the Markov chain, which determines the proportion of vehicles with different car-following modes. Third, a fundamental diagram model is deduced to consider the influence of platoon intensity and platoon size of CAVs. Then, numerical experiments are conducted to analyze the impact of relevant factors (i.e., platoon intensity, platoon size, platoon management strategy, and penetration rate of CAVs) on the fundamental diagram. Finally, the simulation experiments are designed to verify the theoretical model. The results show that (1) platoon intensity, platoon size, and the penetration rate of CAVs have a positive effect on improving traffic capacity; (2) a platoon size of 2–4 with a fixed platoon intensity of 0.5 significantly improves the capacity of mixed traffic flow.