Web services is a popular distributed systems technology and its effectiveness and efficiency rely badly on the underlying protocols. And web service protocols are designed in XML formats so the message structures within are quite different from the conventional ones. Therefore, the well-established formal verification techniques for conventional protocols, which have gained substantial achievements in practice, cannot be applied directly to them because of the inherently different syntax. In this paper, we propose a justification-oriented and automatic formal approach to verify, in the standard Dolev-Yao model, concerned security properties expressed as epistemic notions,i.e., logic of knowledge, for web service protocols, based on a fault-preserving mapping tool called SuD (SOAP under Dolev-Yao). Our approach is significant because, instead of finding flaws in finite number of protocol sessions, the pecifications we are to verify can hold in arbitrary number of sessions.