Abstract: In order to solve the problems,i.e.,fine particles diffusion,pressure drop of boiler bed,low temperature of outlet flue gas and insufficient output,the vortex finder of a 300 MW CFB boiler was optimized and retrofitted.The separation efficiency of separator with vortex finder optimized and its influence on operation effect of boiler were verified by CFD method.The vortex finder was optimized by increasing the insertion depth of vortex finder to reduce the particle escape,reducing the diameter of vortex finder and eccentric layout of necking,lessen particles entering the exhaust pipe The CFD theoretical calculation and fly ash particle size distribution test show that the pressure drop of the separator increases by ca.370 Pa after optimizing the vortex finder shape,and the total separation efficiency of the separator increases to 99.45%,which Increases by 0.54%.The separation effect is good and the proportion of small particles is high.The average bed temperature decreases by 20 ℃ and the differential pressure of upper furnace increases by 0.3 k Pa after retrofitting the vortex finder.The median diameter of fly ash particles decreases from 23 μm to 12.6 μm.The boiler efficiency is improved and the urea consumption is reduced by 50%-70%.The theoretical calculation and field test prove that the design is reasonable and effective.