A speed perturbation model is proposed in this paper for the heterogeneous platoon to measure the interaction between vehicles and investigate the propagation laws of perturbations. Then, a ratio is defined to quantify the relationship between the communicating information impact (CII) and the car-following behaviour impact (CFI). A modified IDM is employed to evaluate the role of communicating information in perturbation propagation. Results show that communicating information can suppress the amplification of perturbation. Under the stable environment, the interaction between vehicles decreases with frequency. There is a critical frequency $ {\omega _c} $ ω c (value of 0.55 in this case) that distinguishes the relationship between the CII and CFI. When the frequency is smaller than $ {\omega _c} $ ω c , the CFI is larger than CII; otherwise, the CII dominates the interactions between vehicles. Under the unstable environment, the interaction between vehicles increases first and decreases then, where the vehicles are mainly affected by the car-following behaviours.