Single-atom metal catalysts (SAMCs) have high catalytic activity, but mass production of SAMCs with high metal loading remains challenging. In this work, a two-step and one-pot strategy is presented to prepare mesoporous carbon nitride (CN)-based Cu single-atom catalysts (Cu-CN-x, where x refers to the metal loading in wt %) with ultrahigh metal loadings (e.g., up to 26.6 wt %), in which the mixture of urea and copper chloride is first heated at 180 degrees C and then calcined at 550 degrees C. Extended X-ray absorption fine structure analysis demonstrates that a Cu single atom is doped into the skeleton of CN via replacing one carbon atom and bonding with three nitrogen atoms. The resultant Cu-CN-x catalyst displays excellent performance and high stability for catalyzing the reaction of terminal alkynes with atmospheric carbon dioxide, much better than the best reported catalyst, synergistically attributed to both the isolated Cu single atom and porous structure of the support. Density functional theory calculation shows that the reaction between CO2 and deprotonated phenylacetylene is energetically exothermic on Cu-CN with a reaction energy of about -0.27 eV and an energy barrier of +0.85 eV. This synthetic strategy paves a universal way for mass production of SAMCs with high-density metal loadings.