Timely design, development and marketing of new service with creative and innovative features are essential for service firm. Context-aware service, in which contextual information is used to adapt the behavior of service automatically to user’s situation and need, is recognized to open up new opportunities for the service creation and service innovation. However, despite of technological possibility, context-aware service is still in its infancy. Lack of systematic methodology for service logic design is one of problems limiting the usage of contextual information to greatest extent. In this paper, we propose petri net as formalism for building a logic model of context-aware service. We make a further study about the dynamic behaviors and properties of the context aware service model, such as deadlock, invalid transition and inconsistency, etc. On this basis, we present a scenario-based method to design context-aware service model. A guidance system for elderly with dementia is designed by this method and the result has partly proved the effectiveness of our model and algorithm.