The reaction mechanisms of the 1Σ+ ground state of YS+ with oxygen-transfer reagent: YS+ +CO2→YO+ +COS in the gas phase has been studied by using density functional theory (DFT). It is found that the reaction proceeds via two four-center transition states (TS1 and TS2) with a cyclic complex (b) locating between them on the reaction potential surface. The activation barriers of the two transition state are -8.3 and 2.1 kcal/mol, respectively, at B3LYP/6-31+G* level plus ZPE relative to the reactants. The second reaction step via transition state TS2 should be the rate-determining reaction step. The similarities and differences between YS+ and ScS+ for this type reaction were also discussed.