The conversion of CO2 is very important for alleviating environmental and resource crisis. Herein, we have systemically investigated a family of emerging 2D metal-organic frameworks (MOFs) utilizing density functional theory (DFT) calculations. The 2D MOFs possess characteristics of single-atom catalysts (SACs), and present higher maneuverability and practicability than modifying materials by doped single metal atoms. Our results show that the Mo-based MOF presents the promising capability of CO2 activation and reduction under ambient conditions. The value of energy cost for CO2 selectively reduced to methane is low with a value of only 0.42 eV. Moreover, the energy input can be further decreased to 0.27 eV with MoO based MOF. These energy inputs are substantially lower than classical pure metal catalysts and some novel SACs based on noble metals. This study provides insights at the atomic level that novel Mo-based MOF, in which molybdenum is an earth-abundant metal, can be used as a cost-effective single-atom electrocatalyst for CO2 reduction at mild conditions.