Abstract: This paper proposed a secure transmission scheme based on the national commercial cryptographic algorithm to address the security issues faced by high-speed railway Electric Multiple Units (EMU) onboard data during transmission. The paper used algorithms such as SM2, SM3, and SM4 for data encryption, digital signature and authentication, and message summarization to enhance the confidentiality, integrity, authenticity, and non-repudiation of high-speed train onboard data transmission, ensured the security of keys during the generation, storage, use, and destruction processes through key lifecycle management. This solution can provide technical support for enhancing the security protection capability of on-board data transmission for EMU.