With the new technologies including big data, cloud computing, the Internet of Things (IoT), mobile Internet, and Artificial Intelligence (AI) coming into widespread in the secondary distribution system, it makes the network boundary more fuzzy, and the network security risk points and exposed surfaces significantly increase. Therefore, the traditional boundary-based security protection model has been unable to meet the protection needs. As one of the most popular security concepts at present, the zero trust mechanism can achieve the dynamic protection of information intranets. Based on the concept of zero trust security and software defined perimeter (SDP) technology, this paper designs and implements a security scheme suitable for the secondary distribution system and proposes a novel identity authentication model which can solve the problems of port exposure that existed in the traditional authentication scheme. In addition, the model applies the SM9 identification algorithm to reduce the computational cost of the encryption and decryption in the proposed scheme. Finally, the performance analysis demonstrates that the proposed scheme is effective and suitable for the secondary distribution system which can effectively resist multiple types of network attacks.