Embodiments of the present invention disclose a method, an apparatus, and a system for establishing a security context and relates to the communications field, so as to comprehensively protect UE data. The method includes: acquiring an encryption algorithm of an access node; acquiring a root key and deriving, according to the root key and the encryption algorithm, an encryption key of the access node; sending the encryption key and the encryption algorithm to the access node, so that the access node starts downlink encryption and uplink decryption; sending the encryption algorithm of the access node to the UE so as to negotiate the encryption algorithm with the UE; and instructing the access node to start downlink encryption and uplink decryption and instructing, during algorithm negotiation, the UE to start downlink decryption and uplink encryption.