The stability of platoon directly determine the safety of the road. For the platoon group, “stability” means that the structure of the platoon is relatively stable when it is disturbed, and the interference will not be amplified. Therefore, this paper studies the internal dynamic characteristics of platoon when it is disturbed. By the analysis, the variation of the internal spacing is consistent with the damping motion, and extensive simulation experiments are conducted by using SUMO(simulation of urban mobility), which validate the analysis conclusion. In addition, we also discuss the influence of different acceleration on the under damping, and sum up the relationship between the acceleration and stability.