With the ever-increasing application of secondary equipment，the integration of secondary devices will be the trend of smart substation.Different schemes for the devices of the process level，bay level and station level are discussed，with emphasis on the integration of the merging unit（MU）and intelligent terminal in the process level.The integration of the protective，measuring and control devices，the concentrated protective devices，the integration of measuring and control devices，the multifunction measuring and control devices，the intelligent components，the integration of the fault recorder and network analyzer are elaborated.The integration supervision and control system at the station level is described.The current application of the different integrated devices in the smart substation is reported，the scheme for the integration of the merging unit and intelligent terminal，the integration of the protective and measuring control devices are recommended for trial application only on the premise that the voltage is 110 kV and below.It will be useful to the construction of smart substations.