There are increasingly prominent communication security problems in distribution network terminals. Thus the security technology requirements of a feeder remote terminal communication system are studied, and the security architecture of a new feeder remote terminal unit (FRTU) is designed. It implements three-layer architecture of a distributed network protocol (DNP3), highlighting application layer objects, variants, groups and security data application classification. This paper focuses on the classification and modeling of FRTU security data, and gives the mapping from FRTU data to the DNP3 security function. A protocol model that conforms to security authentication consistency is designed to effectively solve the security vulnerability of the FRTU. Finally, through the authentication of an international authority and security test, it is proved that security authentication (SAv5) meets the consistency standards such as authentication and encryption. This provides a reference for the secure access of distribution network terminals. This work is supported by the National Key Research and Development Program of China (No. 2018YFB0904900 and No. 2018YFB0904903).