Metal doped MoS2 monolayer as single atom catalyst (SAC) for CO oxidation has been investigated using spin-polarized density functional theory (DFT) calculations. After carefully screening of several normal non-noble metals, Al doped MoS2 is proven to be the most promising SAC. In Eley-Rideal (ER) mechanism, the energy barrier of the rate-determining step (RDS) is as low as 0.19 eV, which is lower than that of Langmuir-Hinshelwool (LH) mechanism (0.39 eV). It is worth to note that the energy barrier of ER mechanism is lower than most SACs, including those graphene based materials. Very interestingly, in the last step of the ER mechanism, the second CO2 cannot be dissociated spontaneously because of the strong interaction between CO2 and Al-MoS2 with the E-ad of -0.78 eV, until another O-2 was adsorbed to weaken their interaction resulting in the energy barrier of 0.01 eV. Then the left adsorbed O-2 will continue the reaction via ER mechanism. Our results demonstrate that Al-MoS2 is a promising SAC for CO oxidation.