With the development of energy Internet and 5G communication technology, the scale of power IoT is increasing, and there is a lack of effective authentication means for cross domain data exchange. The requirements of power IoT terminal cross domain authentication are analyzed and the side chain technology and its implementation mechanism in the cross chain technology of blockchain are discussed. Then, a cross domain authentication scheme of power IoT based on side chain technology is proposed. Firstly, the cross chain authentication architecture based on side chain technology is established, and the block data structure and two-way peg authentication information transfer model are designed. The authentication credentials are generated by public key certificate and digital signature, and the timeliness of authentication information is ensured by time stamp. Then, the cross domain authentication process of power IoT terminal based on side chain technology is proposed, which realizes the authentication certificate from application domain to authentication trusted delivery of the domain. Through the interaction and sharing of main chain and side chain data, the authentication scheme has the characteristics of cross domain effective, tamper proof and distributed consensus. Finally, simulation experiments are carried out in the application scenarios of distribution automation system to verify the feasibility of the scheme.