In order to ensure the Software Defined Network (SDN) flow security access, this paper puts forward a SDN terminal access control system through the deep study on the terminal security access solution in the existing SDN. This system combines the traditional terminal access control technology with the new SDN network and mainly implements some functions such as user’s identity authentication, terminal security status assessment, user’s service authorization, QoS control, and so on. Simultaneously, the system security is analyzed in detail. The network simulation in Mininet is carried out in combination with the RYU controller secondary developed so as to complete the experiment of access control function and communication delay performance. The results show that the security access control system of SDN flow has flexible access control security strategies and then can detect the threats from the insecure terminal access in SDN. This can not only realize the identity authentication of users but also ensure the security of access terminal, and thus realizing the access authorization of terminals in different safe states.