The reaction mechanism of the ground state 1Σ+ of ScS+ with oxygen-transfer reagent: ScS+ +H2O→+ScO+ +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 ECPs of Lan 12dz for Sc. The reaction is proceeding via two steps with seven stationary points (reactants, three intermediate complexes: (Ⅰ), (Ⅱ) and (Ⅲ), two transition states: TS1 and TS2, and products) on the reaction suface which involved two 1, 3-hydrogen-shift reactions from oxygen atom to sulfur atom via a four-center transition state, respectively. The activation energies of the two steps are 25.3 and 30.2 kcal mol-1, respectively, at MP4 (SDTQ)/6-31G**//MP2/6-31G** level plus zero-point energy, which indicates that the second reaction step is the rate-determining step and the theoretical rate constants based on the transition state theory (TST) with Wigner and Eckart tunneling correction are 2.45 and 42.02 (in 10-10 s-1), respectively, for the forward reaction and 0.003 and 0.048 (in 10-10 s-1) for the reverse reaction.