SO3　in　coal-fired　flue　gas　could　cause　negative　effects　on　the　operation　of　power　plants　and　the　atmospheric　environment.　To　further　understand　the　SO3　emission　from　coal-fired　power　plants,　SO3　was　sampled　by　isopropanol　absorption　method　at　the　inlet　and　outlet　of　each　air　pollution　control　device　of　a　300MW　ultra-low　emission　unit.　The　migration　and　removal　characteristics　of　SO3　was　analyzed.　The　results　showed　that　both　of　combustion　process　and　selective　catalytic　reduction　(SCR)　could　converted　SO2　into　SO3.　The　mass　concentration　of　SO3　generated　in　the　combustion　process　accounted　for　0.86%　of　SO2　and　the　conversion　rate　of　SO2/SO3　in　SCR　was　0.45%.　The　SO3　concentration　was　reduced　by　5.7%　with　an　air　preheater.　The　removal　efficiency　of　SO3　by　electrostatic　precipitator　(ESP)　was　not　satisfactory　because　the　temperature　of　the　flue　gas　in　ESP　was　above　110,　which　resulted　in　a　less　condensation　amount　of　H2SO4　acid　mist.　The　removal　efficiency　of　SO3　by　the　two-stage　desulfurization　tower　was　81.3%,　which　was　30%-50%　higher　than　that　of　single　stage　desulfurization　tower.　The　SO3　removal　efficiency　by　wet　electrostatic　precipitator　(WESP)　was　23%.　The　SO3　emission　mass　concentration　of　coal-fired　power　plant　was　2.025mg/m3　with　an　emission　factor　EFcoal　of　0.034kg/t.　©　2020,　Chemical　Industry　Press　Co.,　Ltd.　All　right　reserved.