CuO has been demonstrated to be effective for converting CO into value-added products. However, the mechanism of the carbon dioxide reduction (COR) on the most stable surface, CuO(1 1 1), is still under debate. Additionally, how to improve its activity and selectivity is a challenging issue too. In this work, we unravel that COR can occur before CuO reduction (CuO-R) when the applied potential is below −0.44 V and doping can improve its catalytic performance based on first-principles calculations. The pure CuO(1 1 1) surface shows high activity and selectivity for the production of formic acid (HCOOH). However, the performance of COR deteriorates on the reduced CuO(1 1 1). Doping p-block elements (Al, Ga, In, Tl, Sn, Pb, Bi) is proven to be a workable strategy to improve its catalytic performance by suppressing hydrogen evolution reaction (HER). Importantly, Ga-CuO exhibits the favorable bonding strength for *OCHO, which is responsible for the optimal catalytic activity (−0.18 V) among other p-block elements. Our calculations thus provide an insight into CO reduction mechanism of CuO(1 1 1), favoring rational design of CuO-based catalyst.