Based on an analysis of the current security scheme for public key infrastructure and certification authorities（PKI/CA） in the power dispatching data network and focusing on the contradiction between real time and security， a novel security scheme for identity authentication and key agreement is proposed. By improving the digital signature and key agreement algorithm based on discrete logarithm in a finite field， the identity authentication and key agreement are merged into one session， and the security scheme is made independent of the third party on-line certificate system. Meanwhile， the redundant functions of the current virtual private network（VPN） framework are cut down to replace the traditional PKI/CA system. The proposed security scheme for the power dispatching data network guarantees both security and real time.