A method for implementing security communication. A bit for representing CI is added, and a judge process for the CI and encryption algorithm supported by both the current subscriber and network is added. If the CN supports more than one encryption algorithm, if the CI is 1 and a standard encryption algorithm is supported by both UE and CN, the standard encryption algorithm is determined as the encryption algorithm for security communication: otherwise, the communication is disconnected; if the CI is 0 and a self-developed non-standard encryption algorithm is supported by both UE and CN, the encryption algorithm is determined as the encryption algorithm for security communication; otherwise, the communication is disconnected. If the CN only supports the standard encryption algorithm, if this algorithm is also supported by UE, this standard encryption algorithm is determined as the encryption algorithm for security communication directly; otherwise, the communication is disconnected.