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Using quantum chemistry methods, the mechanism, structure‐activity‐relationship, and water effect in the reactions of criegee intermediates (CIs) with SO2 to form SO3 and aldehydes was investigated. Result indicates that S atom added to the terminal O atom in CIs, and one of the O atoms in SO2 added to C center leading to a five‐membered‐ring intermediate, in which the C‐O and O‐O bond cleavage subsequently to give out SO3. Although the barriers reach to be around 90 kJ/mol, the relative energy of transition states is lower than the initial reactants, and thus, this channel is feasible to undergo. With one H atom in CH2OO substituted by methyl group, energy barrier increases slightly; while methyl replaced by larger n‐alkyl group (from C2H5 to n‐C5H11), the energy converged to be 91 kJ/mol. When 1–4 water molecules take part in the reactions of CH2OO, anti‐/syn‐CH3CHOO and (CH3)2COO with SO2, hydrogen‐bond will be formed between H2O molecules and intermediates or transition states. Result shows that three water molecules is feasible for the reactions of CH2OO and syn‐CH3CHOO with SO, while two H2O molecules give out the lowest barrier in the reactions of anti‐CH3CHOO and (CH3)2COO with SO2.