The main objectives are to explore the possible SO₂ conversion reactions under the condition of calcium chloride absorption method (CCAM) and the effects of these reactions on SO₃ measurement by the CCAM. The vertical-horizontal tube furnace and variable-controlling approach were used to study the influences of water vapor, oxygen atmosphere, temperature, reactant concentration on the chemical reaction between SO₂ and CaCl₂. On this basis, the possible chemical reaction mechanisms of SO₂ and CaCl₂ and the effects on SO₃ measurement were analyzed theoretically. The results showed that SO₂ could not undergo the homogeneous reactions in the absence of CaCl₂. Under typical conditions of CCAM, only when H₂O and O₂ existed simultaneously, an obvious chemical reaction between SO₂and CaCl₂could occur. The conversion rate of SO₂ can be improved with the increase of temperature and reactant concentration and the conversion rate is 2% to 8% approximately. Finally, the theoretical analysis demonstrated that the heterogeneous reaction was extremely week at 200 ℃and under typical sampling atmosphere, and this reaction was severely inhibited in the presence of high concentration of gaseous H₂SO₄. Therefore, the effect of SO₂ interference on SO₃ measurement could be ignored, which proved the feasibility of the CCAM to some extent.