The reaction mechanism of the Y+ cation with CH3CHO has been investigated with a DFT approach. All the stationary points are determined at the UB3LYP/ECP/6-311++G** level of the theory. Both ground and excited state potential energy surfaces are investigated in detail. The present results show that the title reaction start with the formation of a CH3CHO-metal complex followed by C-C, aldehyde C-H, methyl C-H and C-O activation. These reactions can lead to four different products (Y+CH4 + CO, Y+CO + CH4, Y+COCH2 + H2 and Y+O + C2H4). The minimum energy reaction path is found to involve the spin inversion in the different reaction steps, this potential energy curve-crossing dramatically affects reaction exothermic. The present results may be helpful in understanding the mechanism of the title reaction and further experimental investigation of the reaction.