The testing of bay level devices is one of the emphases of the smart substation on-site debugging. At present, the debugging of the interval layer devices mainly focuses on the test of single device, however, the interconnection relationship between the interval layer devices cannot be truly reflected. The interconnection test between the interval layer devices is mainly studied. According to the characteristics of the equipment including the relay protection, measurement and control devices, merging units and intelligent terminals in the bay level, three types of interconnection test principles are researched, including the interlocking test aimed at GOOSE message, longitudinal channel interlocking test as well as single interval joint black box test. Based on the test principle and requirements, the hardware and software framework of the automatic test system for the interval unit is constructed, and the test template intelligence generation technology for different test methods is designed. Through the research of the automatic test technology for the digital substation interval unit, the interconnection relationship of the field devices becomes more realistic in the on-the-spot debugging, and the debugging quality is greatly improved under the premise of ensuring the efficiency, which reduces the security risks of the power grid, a new direction of development for sub-system test is put forward.