Abstract: For determination of SO₃ in high content in production of phosphoric acid by wet process, a self-designed automatic analyzer system was fabricated and modification of reaction condition for turbidimetric determination of BaSO₄ suspension was made. In the determination by the automatic analyzer, the sample was pumped into the AC coil where the sample was acidified and diluted. 25 μL of the sample was taken by a volumetric vessel and injected into the reaction system of RC₁ and RC₂. In reaction coil RC₁, SO₃ in the sample was reacted with BaCl₂ in mixed reagents solution Ⅰ to form BaSO₄ suspension; and through co-reaction of 2 other reagents in the mixed reagents solution Ⅰ, i.e. PEG-4000 and ethanol, coagulation of BaSO₄ particles was avoided and interaction among particles were reduced, leading to less formation of BaSO₄ and lowering the sensitivity of determination and raising of stability. The reaction mixture was then passed through the reaction coil RC₂ where complexation reaction was occured between the BaSO₄ and Na₂EDTA present in mixed reagent solution Ⅱ, and BaSO₄ was simultaneously wrapped up by Na₂EDTA, thus leading to further lowering of sensitivity. Finally, the reaction solution was passed to the flowing cell of the spectrophotometer, and the absorbance of the BaSO₄ suspension was measured at the wavelength of 680.0 nm. By the optimizing test, the mass fraction or volumic fraction of the reagents found were as follows:in mixed reagent solution Ⅰ, BaCl₂ 4%, PEG-4000 3% and ethanol 2%; in mixed reagents solution Ⅱ, Na₂EDTA 6%, aq. ammonia 6%. Five substantial samples were analyzed by the proposed method and checked by the gravimetric method, the results obtained by the 2 methods were checked quite well, giving relative deviations within ±2%. Interferences of macro amounts of coexisting PO₄^3- and F^- were eliminated by controlling the acidity at pH 2.