Abstract: Flow field characters have influential impacts on the improvement of desulfurization efficiency and system stability in the circulating fluidized bed semi dry desulfurization system,among whichthe uniformity and stability of the flow field are two key points of the whole system. Only when the design of fluidized bed and conveying equipment are reasonable,the stable and efficient operation of the desulfurization system can be guaranteed. Using CFD method and taking a semi-dry desulfurization plant of 300 MW coal-fired unit as the research objectives,and based on the calculation of the flue gas field in the desulfurization tower area and flue behind the dust collector,the non-uniformity of the flue gas flow field in the semi dry desulfurization system and improvement methods were investigated. Two models were established in the experiment. Model A is from the header of the 4 trumpet outlets of the pre-dust collector to the entrance of the bag dust collector,which is the main desulfurization reaction area. Model B is from the outlet of the bag filter to the inlet header of the induced draft fan and the main purpose of this area optimization is to reduce system resistance. Computational fluid dynamics simulation has been taken for analyzing placement and the number of guide vane as factors affecting flue flow field,velocity vector and pressure field. The result of optimization showed that the uniformity of the flow field can be improved by the application of the expansion header at the outlet of the pre-dust collector in model A. However,it is necessary to consider the ash accumulation caused by the reduction of the expansion flow rate,adjust the design of the ash accumulation load of the flue and steel structure and improve safety margin. A deflector blade is installed at the bottom of the desulfurization tower,which can effectively solve the bias problem and ensure the uniformity of the flue gas velocity entering the venturi,and the system resistance is also well controlled. The total resistance is around 2 200 Pa. The main optimization element of the bag flue outlet flue( model B) is to reduce the system resistance. The guide vane at the elbow 1 can effectively improve the speed distribution and reduce the resistance of the flue. When the number of guide vanes is 1-2,the effect is poor. When the number of guide vanes exceeds 3,the resistance basically reaches the optimal value and the air flow distribution is apparently improved. The guide vanes at the elbow 2 have little effect on reducing the resistance and improving the uniformity of flue gas flow field.