Graphitic carbon nitride (g-C3N4) is a kind of two-dimensional material with unique properties, and can be facilely prepared from low-cost precursors. It has promising application in CO2 capture. However, the low alkalinity of nitrogen species in g-C3N4 makes its interaction with CO2 very weak. Aiming to promote the use of g-C3N4 for CO2 capture, a class of amine-functionalized g-C3N4 adsorbents were designed in the present work. The amine-functionalized g-C3N4 adsorbents were prepared by physical impregnation of polyethylenimine (PEI), which contains many amine groups and exhibits extremely low volatility, into the stacked pores of g-C3N4. It is found that, after the amine functionalization, the performance of g-C3N4 for CO, capture (including capacities and selectivities) are significantly improved. The highest CO2 capacity of PEI-g-C3N4 composites is 3.77 mmol/g when determined at 100 degrees C and ambient pressure, being superior to most of those of other g-C3N4 materials and solid amines reported in the literature. Furthermore, the CO2 adsorbed by PEI-g-C3N4 composites can be facilely stripped out by concentration swing, and the loss in CO2 capacity is negligible after 10 cycles of adsorption and desorption.