To promote the development level of urban sustainability, more and more cities have been paying attention to the improvement of public transportation. City managers intend to attract people from private cars to public transport by improving the service level of urban public transport. The operational reliability of bus lines plays a crucial role in maintaining high-level service of public transportation. Previous studies have focused on the service level of the whole line by investigating the overall stability of departure punctuality, line running time, and punctuality. This study aims to clarify the relationship between the whole line and sites in terms of the influence on the operational reliability of bus lines. This study proposes a bus route reliability evaluation method, based on copulas connect function, and the actual line data are taken as a case study. The results show that the method reveals the relationship between the overall line and the interstation paths in the context of operational reliability. This study finds the interstation path, which is more critical to the whole line. This method can provide a good reference for the diagnosis and evaluation of line operation and the optimization of bus routes. The results can support the sustainable development of public transportation.