Abstract: Describes the design of a high security and adaptability single sign on system. By means of PKI and middle part technology, a SSO system based on PKI (KSSO) is implemented. This system can log on all the authorized applications securely and transparently through implementation of log on proxy of authorized applications, SSL based identity authentication, and the concentrated management of log on credential library. To help the implementation of SSO, independent CA is built. To use this SSO system, no change is needed to current applications.