| In suburban rail transit line, the operation of fast-slow trains can be well adapted to uneven spatial and temporal distribution of the passenger flow. However, due to the use of non parallel operation diagram, the proportion of fast and slow vehicles, the distribution and quantity of the overtaking station, the interval of train departure and so on, all have different influence on the carrying capacity of the line. In order to study the influence mechanism and internal relationship of carrying capacity with different parameters of suburban line under fast-slow mode, a calculation idea was put forward, and then the calculation and expression of the carrying capacity was established by considering the quantity of overtaking and the proportion of fast and slow vehicles. Shanghai Metro Line 16 being taken as an empirical study, the proposed methods were proved effective. This study results contribute to a theoretical reference for the calculation of the carrying capacity of suburban line under fast-slow mode.