A type-inference-based formal method is proposed for verifying an ad-hoc security routing protocol in this paper. A calculus, called NCCC (neighborhood-constraint communication calculus), is defined to specify the protocol. The security property of the protocol is described with typing rules in a type system. Based on the Dolev-Yao model, an attacker model, called the message set of protocol format, is refined. At last, the simplified version of SAODV (secure ad hoc on-demand routing protocol) is verified with this method. With the type- inference-based formal method, not only is the security of protocols verified, but also the attacke examples are predicted. The complexity of inference is reduced significantly for refining the message set of protocol.