The key technologies for protection automatic test system are researched, including the interface characteristic and reusability of testcase. A method of establishing relay protection automatic test system based on relay protection test device is put forward. Design method, system structure, software design and scheduling flow of main task are introduced in detail. The system applies to all kinds of relay protections of smart substation. The realization approach and construction of the automatic test system are referential to other engineering practices in smart substation, such as debugging and acceptance test.