Due to heavy traffic at peak times, it is necessary to ensure that trains have an absolute safety braking distance. For this problem, this paper not only analyzes various factors that influence a metro's stopping time, but also analyzes a time model of a metro's stopping in the station. Secondly, based on the model and combined with the physical process of a metro's approach, this paper calculates the braking distance of oncoming trains. Finally, a novel relation between the number of passengers and the braking distance of an oncoming metro is established. Theoretical analysis and simulation experiments indicate that the braking distance of an oncoming metro can be effectively calculated according to the number of passengers on the platform.