Two-dimensional (2D) material-based single-atom catalysts (SACs) have demonstrated their potential in electrochemical reduction reactions but exploring suitable 2D material-based SACs for the CO2 reduction reaction (CO2RR) by experiments is still a formidable task. In this study, theoretical screening of transition metal (TM)-doped graphitic carbon nitride (g-C3N4) materials as catalysts for the CO2RR was systematically performed based on density functional theory (DFT) calculations. An indicator for the selective formation of one carbon (C1) products was developed to screen catalysts that are active and selective in the CO2RR. The results indicated that Ti- and Ag-g-C3N4 demonstrate excellent catalytic activity and selectivity for the formation of CO and HCOOH, with limiting potentials of -0.330 and -0.096 V, respectively, while Cr-g-C3N4 exhibits the highest catalytic activity for yielding CH3OH and CH4 (-0.355 and -0.420 V, respectively), but none of the screened catalysts have been identified as ideal candidates for the selective production of CH3OH and CH4. Furthermore, Bader charge analysis suggested that excessive electron transfer from TM leads to stronger adsorption of intermediates and high limiting potentials, which subsequently result in lower catalytic activity. This work provides theoretical insights into the effective screening of active and selective 2D material-based SACs which has the potential to significantly reduce the time and resources required for the discovery of novel electrocatalysts for the controlled formation of various products.