Joint　surfaces　widely　exist　in　mechanical　assembly　structures,　the　local　stiffness　and　damping　have　important　effect　on　the　whole　dynamic　analysis　of　the　structure.　In　this　study,　proposed　a　normal　contact　damping　forecast　model　and　analyze　its　damping　characteristics,　based　on　the　microcosmic　mechanism　between　joint　surfaces　and　citing　the　domain　extension　factor　and　unit　area　radio,　combine　with　the　modified　fractal　theory　and　energy　dissipation.　The　simulation　results　show　that　the　damping　characteristic　curve　appear　inflection　point　when　the　fractal　dimension　value　D　is　1.428.　When,　the　normal　contact　damping　increases　with　the　increase　of　fractal　dimension　but　decreases　with　the　increase　of　contact　area.　When,　the　normal　contact　damping　increase　with　the　increase　of　contact　area.　Since　the　reaction　of　real　contact　area　is　relate　to　the　change　of　the　load,　the　relationship　between　the　normal　contact　damping　and　the　contact　area　can　accurately　predict　the　performance　of　assembly　structure　to　a　certain　extent,　the　existence　of　the　inflection　point　also　provide　a　reference　for　the　optimization　design　of　assembly　structure.