The stability of transmission section has great influence on whole power system stability. In offline simulation, the transmission section analysis is mainly analyzed by person. However, it should be analyzed automatically in order to suitable for power system network change when used in online stability assessment. Based on the characteristic of actual power system, the paper presents a transmission section automatic identification method based on sub-network combination, and the types of transmission line include high voltage electromagnetic loop section and non-electromagnetic loop section. Firstly, the whole power network is simplified according to substation and power supply area; then, starting from one subnet and considering one or more subnets automatic combination, the high voltage non electromagnetic loop sections are found from all the power out lines of one or more subnet; starting from one high voltage substation of power supply area and considering one or more substation automatic combination, the high voltage lines of electromagnetic loop sections are found from all the power out lines of one or more substation. Finally, the north China and central China interconnected power system is used to verify the validity of the method. The transmission section searching is an important basis of online power system stability assessment and has great practical application value.