Abstract: This paper analyzed X.509 authentication protocol with Authentication Test in Strand Space. The conclusion is more concrete than that achieved using BAN Logic, and the proving process is simpler and ocularrer than the traditional method of constructing the set and finding the M-minimal element in the set. Then we came up with an improved version based on the analysis conclusion, and proved with Authentication Test that the improved protocol can realize the correctness of authentication while keeping the secrecy and integration goal of the original protocol.