To optimize the allocation of remote terminal unit of distribution automation system, which meets the requirement of different network structures and different power supply reliability, the sensitivity between power supply reliability and the number of feeder terminal units (FTUs) is analyzed. The method of middle voltage distribution network FTUs optimal configuration are proposed based on the consequences of failure mode and effect analysis. On the basis of considering technical, economic and scalability factors, line failure outage time and average service available index (ASAI) are analyzed in three cases: line without configuration of terminal unit, line with configuration of monitoring terminal unit only and line with mixed configuration of monitoring terminal unit and controlling terminal unit. The generic expression of ASAI is deduced, as well as the number of FTUs. Example demonstrates the rationality and practicality of research methods.