The semi - dry desulfurization technique of circulating fluidized bed (CFB) is the absorption of SO₂ in flue gas conducted by the injected high - pressure water and the powder absorber based on the powder absorber in the tower under the boiling bed state. In order to optimize the internal structure of the absorption tower and the uniformity of flue gas flow, improve the effect of the process water lance and the fluidity of the material in the system, combined with a sintering gas CFB semi - dry desulfurization project of an iron and steel enterprise, using 3D simulation technology to model the desulfurization tower, analyze the structure of the absorption tower, the type of lance, the absorber inlet and return position. The computational fluid dynamics (CFD) simulation technology is used to systematically simulate the flow rate uniformity of the tower body flue gas, thus, the air flow distribution effect of the additional deflector plate and turbulent ring is verified. The results show that: 1) in the optimized model simulation, the flue gas bias flow phenomenon has been significantly improved, and the relative standard deviation of the average flue gas flow rate through the venturi center section has decreased from 8.2% to 1.8% ; 2) the high pressure reflux lance can obtain a larger water mist particle size, which is more conducive to cooling flue gas and prolonging the reaction time ; 3) the absorber inlet and return material into the tower position set at the lower side of the venturi inlet can reduce the generation of calcium chloride and avoid material agglomeration, adhesion to the tower wall and the decrease of material fluidity; 4) the desulfurization efficiency is increased from 93.3% before the ultra - low emission retrofit to 98.3% after the retrofit, and the flue gas outlet SO₂ mass concentration is kept below 25 mg/m³.