Compared with the previous determination methods relying on the section imbalance coefficient or the premise of setting the number of stations, this paper adds the algorithm of the OD volume ratio of the section train passenger flow under all section lengths by traversing on the basis of improving the traditional OD proportion method, and then combines the marginal effect theory to find the point where the growth slope decreases to obtain the optimal operation section and departure station. The case analysis selected a certain Third Bus Route in Guangzhou with a total of 24 stations and a length of 20.8KM, and calculated the optimal solutions under 23 different section lengths, including 275 combinations. The most suitable section was found through the marginal effect, and the stability of the section determination method proposed in this paper was verified by the judgment and comparison of the sections in the two directions: A fixed section combination can be obtained without the limitation of the number of stations and the artificially set coefficient threshold.