In this work, three-dimensional cobalt doped graphitic carbon nitride was synthesized by molten salt assistant method. XRD, N-2 adsorption, SEM, XPS, UV-Vis spectroscopy, N-2-TPD and Photoluminescence were used to characterize the prepared catalysts. The result shows that the addition of molten salt remarkably changes the morphology of as-prepared catalyst. Cobalt exists as coordinative Co-N bond, which can chemisorb and activate N-2 molecules, and promote the electron transfer from catalyst to the nitrogen molecules. The as-prepared cobalt doped g-C3N4 displays the NH4+ production rate of 5.8 mg.L-1.h(-1)center dot gcat(-1), which is 24 times of bulk g-C3N4. Density functional theory (DFT) simulations show that cobalt doping can not only prolong N equivalent to N bond which activate N-2 molecules, but promote the electrons-holes separation efficiency of g-C3N4. The d orbit of cobalt easily acts as a channel for transferring electrons from the transition metal to the N-2 molecule, which enhances the N-2 photofixation ability.