The reaction mechanism of the ground state 2∆ of TiS+ with oxygen-transfer reagent: TiS+ +H2O→TiO+ +H2S in the gas phase has been proposed and investigated by ab initio methods with 6-31G** basis set for non-metal atoms and the effective core potentials of Lanl2dz for Ti. The reaction is proceeding via two steps with seven stationary points (reactants, three intermediate complexes: (a), (b) and (c) two transition states: TS1 and TS2, and products) on the reaction potential surface which involved two 1, 3-hydrogen shift reactions from oxygen atom to sulfur atom via a four-center transition state. The activation barriers of the two transition states are -54.0 and -65.1 kcal/mol, respectively, at MP4 (SDTQ)/6-31G**//MP2/6-31G** level plus zero-point energy relative to the reactants, which indicates that cationic titanium sulfide is favorable to this type of reaction and the collision rate of the reactants forming the complex (a) should be the main factor that determines the reaction rate.