The structure of a Web form was analyzed, logically optimized and reorganized; and then a formal description of a Web form was given. Based on a new ontology building method, the semantic model of a Web form was built. The model could be used to identify a form，prepare data and submit query automatically. It also facilitates getting the semantic of query results. At last, the process was explained with an example. The model proved to be practical by experiment, and it is useful for automatically querying dynamic Web contents.