This paper describes the security protocol verifier SPVT developed by Objective-Caml. In SPVT (security protocol verifying tool), the specification language is the π-like calculus extended with three appendixes, the Dolev-Yao model is described with Horn logic rules, the π-like calculus model of security protocol is transformed into the logic program model by abstract rules, the security properties are verified based on the calculus of the logic program’s fixpoint, and the counter-examples on security properties are constructed from the process of the fixpoint calculus and the process of the property verification. The simplified Needham-Schroeder public-key authentication protocol is used to exemplify the automatic verification process of security protocol with SPVT, and the results show the validity of the verifier.