This　paper　considers　a　new　modeling　method　and　traffic　state　estimation　for　a　freeway　network.　Firstly,　we　propose　a　new　modeling　method,　which　is　called　Dynamic　Graph　Hybrid　Automata　(DGHA),　to　describe　the　evolution　law　of　the　freeway　network.　In　the　model,　the　traffic　dynamics　are　described　by　the　Modified　Cell　Transmission　Model　(MCTM),　where　the　cells　are　expressed　by　hybrid　automata,　and　multi-mode　switching　expressions　are　introduced　into　the　connections　among　these　cells.　The　proposed　DGHA　model　is　modularized　and　easily-extensible.　Combination　between　cells　is　implemented　automatically　by　an　algorithm.　By　using　the　modeling　method　and　the　algorithm　of　combination　between　cells,　one　can　establish　the　freeway　network　models　with　any　topology　structure.　Secondly,　a　switched　state　observer　is　designed　to　estimate　the　traffic　states　of　the　freeway　network.　Finally,　the　proposed　method　is　applied　to　Beijing　third　ring　freeway,　which　shows　the　effectiveness　of　the　proposed　modeling　method　and　the　accuracy　of　the　state　estimation.