Graphitic carbon nitride (g-C3N4) is a promising photocatalyst for the reduction of CO2 into fuels. However, the reduction mechanism of CO2 using g-C3N4 is not clear in the literature. In the present study, the fixation of CO2 and the formation of carbamate on the nitrogen atom at the edge of g-C3N4 were investigated using first-principles density functional theory. The calculated results shows that two adjacent bare nitrogen atoms at the edge of g-C3N4 could be the activation sites for the proton and CO2 molecule respectively, which are crucial to the formation of carbamate. The calculated energy barrier of carbamate formation is 0.95 eV for a preferential pathway. From studies on these micro processes, we propose a mechanism with proton assistance for the g-C3N4-catalyzed photoreduction of CO2 to CO.