Abstract: To address the challenge that the traditional unified encryption strategies for smart substations cannot simultaneously guarantee both data transmission security and real-time performance, this paper proposes a data classification and differentiated secure transmission technology oriented to smart substations. The proposed technology divides smart substation data into four categories based on data characteristics and service importance, and establishes a differentiated encryption system adapted to different data types. Specifically, an integrated lightweight encryption mechanism based on sm4-galois/counter mode (SM4-GCM) is adopted for control instruction data; and a cooperative encryption mechanism combining sm4-cipher block chaining mode (SM4-CBC) and sm3-message authentication code (SM3-MAC) is applied to sampling data; batch encryption and access control mechanism based on sm9 identity-based cryptographic algorithm (SM9) are used for state monitoring data and management operation & maintenance data. Meanwhile a life-cycle key management system is constructed. Simulation results demonstrate that the proposed scheme outperforms the traditional unified encryption scheme in key metrics such as transmission latency and encryption efficiency. It can effectively resist network attacks including man-in-the-middle attacks and data tampering, thereby meeting the transmission security and real-time requirements for various data types in smart substations.