The　＇chimney　rain＇　problem　can　effectively　be　solved　by　adding　a　secondary　dehydration　device　inside　the　chimney　with　wet　flue　gas　desulfurization　(FGD)　direct-discharged　measures　in　coal　fired　power　station.　In　order　to　analyze　the　changes　of　outlet　moisture　content　and　internal　flow　field,　and　to　optimize　the　structure　of　device　to　match　the　dehydration　capacity　and　working　resistance,　a　computational　fluid　dynamics　(CFD)　method　was　adopted　to　simulate　the　internal　flow　field　of　the　chimney.　Results　show　that　after　installing　a　secondary　dehydration　device　the　outlet　moisture　ratio　decreases　from　111　mg/m3　to　52　mg/m3;　the　working　resistence　increases　in　an　allowable　range,　from　195　Pa　to　699　Pa,　which　is　below　1200　Pa;　the　outlet　velocity　raises　from　17.3　m/s　to　17.6　m/s.　The　smoke　spirals　up　along　inwall,　not　along　central　zone;　the　optimum　angle　of　the　whirl　board　is　42　degrees.　Through　the　comparative　analysis　of　actual　and　simulation　results,　the　effectivity　of　the　device　and　the　feasibility　of　optimum　angle　were　verified,　and　the　basic　laws　and　characteristics　curves　of　internal　flow　field　were　also　obtained.