All existing Web service composition standards remain at the descriptive level, without providing any formal semantics and method for verifying some important properties. Web Service Choreography Interface (WSCI) describes the flow of messages exchanged by a Web service which participates in choreographed interactions with other services. This paper presents an extended Petri net to formalize WSCI, and proposes several nets to represent the activity, process and interface respectively. Our formal model remarkably focuses on the message exchange and the context of WSCI. Furthermore, we give the formal semantics for the context in a subtle manner, i.e., subprocess and exception handling. According to the characteristic of WSCI, this paper proposes some properties just like well-founded, and introduces technique for checking these properties to ensure the correct implementation and deployment of WSCI.