Experimentally well-characterized dual-atom catalysts (DACs), where two adjacent metal-atoms are stably anchored on carbon defects, are recently a hot topic in the catalyst community. DACs have shown some clear advantages in electrocatalysis compared to conventional catalysts and the emerging single-atom catalysts. To design and understand the performance of DACs for electrocatalysis, ab initio calculations have been widely employed. However, most previous theoretical studies used a pristine dual-atom site to analyze the electrocatalytic activity of a DAC. Herein, by analyzing M-M’-Nx-C DAC structures (where M, N, and C represent the metal, nitrogen, and carbon, respectively) with ab initio calculations, our derived surface Pourbaix diagrams show that the surface states of DACs generally differ from a pristine surface at electrocatalytic operating conditions, due to the strong adsorption capacity of a DAC’s unique metal-metal bridge site. This phenomenon suggests that the surface state of a DAC should be considered before analyzing the catalytic activity in electrocatalysis, while the electrochemistry-driven pre-adsorbed molecules generated from the liquid phase may either change the electronic properties or even block the active site of DACs. Based on these results, we add a critical comment to the DAC community: before analyzing the electrocatalytic activity of a DAC, its surface state should be analyzed beforehand.