In this paper, we present a comprehensive computational study on the hydrogen sulfide sensing mechanism in aqueous solution using a pyrylium derivative. The possible sensing mechanisms were investigated under the neutral condition and acidic condition in the gas phase and in aqueous solution. The pyrylium-thiopyrylium transformation under the neutral condition is thermodynamically unfavorable, while it is greatly facilitated in the acidic condition catalyzed by a hydronium cation. In addition, the UV-vis absorption maxima of pyryliums and thiopyryliums were investigated at the TDDFT/B3LYP/6-31G+(d,p) level. The red shift of absorption maximum from unsubstituted pyrylium and thiopyrylium to dimethylamino-subisituted pyrylium and thiopyrylium as well as the red shift seen in the pyrylium-thiopyrylium transformation is interpreted in terms of the molecular orbital theory.