This paper proposed a Kahn Process Network(KPN) based service interaction model which can dynamically establish links between computational service nodes. Three advantages of KPN make it adequate to model service interactions: (1) parallelism and communication mechanism in KPNs, which enable distributed service interaction on internet; (2) KPNs are compositional, which corresponds to the possibility to build bigger behaviors from small ones; (3) KPN can be executed, which ensures a executable service interaction environment for actual application. Under the three advantages above, we propose four kinds of service interaction rules and corresponding interaction events which enrich KPN operations and extend KPN semantics. We design a service description language called SDL to describe both static properties and dynamic interactions of services. The implementation architecture of service interaction model is present. Finally we introduce an application case to show how to describe service interactions using SDL.