Researchers for complex systems become more and more important in modern science. System dynamics has done its significant work for the integration of System Theory and Computer Science in this field. Each dynamic system forms a complex causality network. Now we can use the panweighted network in Pansystems Theory for the dynamic system modeling, and further perform the automatic reasoning on this model. This new ideal may be developed into a deep seated issue in AI. In this article, the method of both modeling and automatic reasoning on a panweighted network in dynamic systems will be introducted together with a simple and typical example in System dynamics. The further extention of this new method will be discussed in other articles.