A token based single sign-on protocol for distribution systems is proposed in this paper. When a user C logs on a system, a centralized authentication server A will authenticate C and issue C a token which is signed by A and includes a session key generated by A as well as a time stamp. C can use the token to access any application server S.S will send the C’s request to the A. Then A will verify the validity of the token. There are two advantages of this protocol: 1) Time synchronization between severs S and the user C is not necessary. 2) All authentication state information such as session key is stored in the token rather than in the memory of A, thus the performance of A can be promoted effectively.We have used SVO logic to do formal analysis of this protocol.