Abstract: Suburban railway links the central city area and suburban areas, but passenger travels on the suburban railway appear obvious uneven spatial-temporal distribution. The study on suburban speed-to-slow train ratio optimization could effectively solve the detention of passengers and save the operation cost. Based on the gray forecasting model, the historical passenger flow data are used to predict the future short-term traffic in different time periods. On this basis, an optimization model of the opening suburban train proportion on a certain day is established according to the predicted passenger flow value, then the branch and bound method is adopted to solve the problem. Finally, Shanghai rail transit Line 16 is taken as an example to carry out a study on the line opening ratio optimization, which could provide a reference for the determination of suburban speed-to-slow train proportion.