Context middleware is an effective way to isolate the complexity of acquiring, processing and using context/situation from context-aware applications. The context middleware design and implementation needs to support different applications, therefore the corresponding context modeling faces more challenges: how to support different requirement patterns, to model situation transitions effectively, and to simplify the using process of context. But most of current works only cover parts of the above challenges. There is no one reference model covering all. To address the issue, we present a layered context reference model (LCSM) for the context middleware to support different requirements of context/situation modeling. Each layer and mapping between layer models is presented in detail, together with the application of LCSM model. We have implemented a prototype to illustrate the effectiveness of the LCSM model to the case. Compared with most relevant context modeling approaches, the LCSM reference model shows good adaptability to different requirement patterns.