Crystal planes of a catalyst play crucial role in determining the electrocatalytic performance for CO2reduction. The catalyst SnO2can convert CO2molecules into valuable formic acid (HCOOH). Incorporating heteroatom N into SnO2further improves its catalytic activity. To understand the mechanism and realize a highly efficient CO2-to-HCOOH conversion, we used density functional theory (DFT) to calculate the free energy of CO2reduction reactions (CO2RR) on different crystal planes of N-doped SnO2(N-SnO2). The results indicate that N-SnO2lowered the activation energy of intermediates leading to a better catalytic performance than pure SnO2. We also discovered that the N-SnO2(211) plane possesses the most suitable free energy during the reduction process, exhibiting the best catalytic ability for the CO2-to-HCOOH conversion. The intermediate of CO2RR on N-SnO2is HCOO* or COOH* instead of OCHO*. These results may provide useful insights into the mechanism of CO2RR, and promote the development of heteroatom-doped catalyst for efficient CO2RR.