Abstract: In the implementing of STEP, the application protocol(AP) provides share model for specific domain, but it limits to operate each other.It is difficult to ensure its conformance and augmentability. By reference of AP214, it is concerned with describing an approach of modeling based on STEP/AP. It defines four levels of product information architecture and relationships between them. Emphasis is on the whole process of modeling.By building model on four levels,the granularity of product information becomes thin increasingly. Finally, the model being appropriate to describe by computers can be obtained.More importantly, it demonstrates how to extend a model and ensure conformance. It is flexible, and potentially supports modeling on application domain.