Community　detection　is　able　to　explore　the　individual　sets　with　the　same　characteristics,　which　is　helpful　for　people　to　understand　the　structures　and　functions　of　the　networks　more　clearly.　In　this　paper,　an　Incremental　dynamic　Community　discovery　algorithm　based　on　Improved　Modularity(ICIM)　is　proposed　to　find　the　dynamic　network　structures,　the　algorithm　uses　improved　modularity　as　evaluation　index　of　the　communities.　Community　structures　are　adjusted　according　to　the　historical　moment　topology　and　the　impact　of　incremental　changes　on　the　belonging　coefficients　of　vertices　neighbors　with　the　change　of　the　nodes　and　edges　in　the　local　area.　Experimental　results　show　that　the　algorithm　can　find　community　structure　effectively　and　timely.