The imperative need to reduce atmospheric CO2 to mitigate global warming and advance sustainable energy systems has driven extensive research. Copper oxide (Cu2O) has emerged as a promising catalyst for electroreduction of CO2, yet its catalytic mechanism remains debated. This study employs a theoretical approach to delve into the intricate details of CO2 electroreduction catalyzed by Cu2O. Utilizing both periodic boundary and saturation cluster models, we explore various reaction pathways, successfully identifying key transition states and intermediates with each model. A comprehensive analysis and comparison of results from both models are conducted. To enhance understanding, we develop a method simulating the impact of an electric field, employed to probe the ongoing reaction. Additionally, we consider the solid–liquid interface by introducing solvent molecules and buffer ions into our analysis. This comprehensive theoretical investigation aims to elucidate the mechanism governing the electroreduction of CO2 catalyzed by Cu2O and analogous semiconductor oxides, providing valuable insights into this critical process.