Electrochemical reduction of carbon dioxide (CO2) to fuels and value-added chemicals provides an intriguing approach to realizing the artificial carbon cycle. The development of low-cost electrocatalysts with high activity, selectivity, and stability for CO2 reduction is of great significance but still a big challenge. Herein, we present a dual-functionalization strategy to synergistically active polymeric carbon nitride (PCN) for CO2 electroreduction by modifying hydroxyl and amino groups on the surface. Compared with unmodified PCN, a 17.1-fold enhancement for CO yield rate is achieved. The optimized ratio of CO/H2 is 0.67, which is in the range for Fischer–Tropsch reactions (0.25–3.34). The experiments and theory calculations propose that the superficial hydroxyl and amino groups both serve as active sites for synergetic activation of CO2. This work may open an avenue for designing other metal-free electrocatalysts for CO2 conversion.