The interchangeability of secondary devices in smart substation is discussed. Technologies needed for interchangeability are analyzed and three different interchangeable modes are summarized on the feasibility, problem and solution. These modes include:replacement of identical products from the same manufacturer, exchange of products with similar function provided by different manufacturers, the interchange of devices with similar function based on the general software and hardware platform. An effective implement scheme to the second mode currently concerned is proposed respectively considering intelligent terminal of merging unit in the process level, protection and monitor devices in the bay level and devices in the station level, which provides useful technologies for promoting implementation project of relay protection devices interchangeability in smart substation.