Recycled　fly　ash　is　considered　as　SO2　removal　agent,　most　study　focus　on　activating　of　SO2　sorbents.　It　is　reported　that　reacting　efficient　increases　in　the　presence　of　steam　in　dry　and　semi-dry　flue　gas　desulhpurization　(FGD).　With　study　of　mechanism　of　semi-dry　FGD,　a　full　set　of　laboratory　FGD　system　was　established,　in　which　SO2　sorbent　were　activated　by　steam　with　temperature　below　180°C　during　conveyance,　and　the　pressure　of　about　0.4　MPa,　characteristics　of　SO2　sorbent　of　various　ratio　of　lime/fly　ash.　Experiments　were　conducted　by　means　of　mixture　with　three　types　of　lime/fly　ash　ratio　as　SO2　sorbents,　testing　results　showed　that　during　steam　conveyance,　part　of　CaO　of　SO2　sorbent　transformed　into　Ca　(OH)2,　and　specific　surface　area　increased　from　7.8688　m/g　to　10.0715　m/g.　It　is　well　accepted　that　enhanced　SO2　sorbent　with　fly　ash　greatly　improve　desulphurization　efficiency.　Study　showed　that　SO2　removal　efficiency　of　86.9%　achieved,　with　lime　to　fly　ash　ratio　of　1:8(wt/wt),　which　increased　12.8%　over　with　lime　as　SO2　sorbent　only.　Test　results　shows　that　SO2　sorbent　activated　by　steam　has　a　significant　effect　on　reactivity　of　SO2　removal　efficiency.