In this work, N vacancy-doped g-C3N4/Ag2CO3 heterojunction catalyst with outstanding photocatalytic N-2 fixation ability was synthesized via in-situ self-sacrificial method. XRD, UV-Vis, N-2 adsorption, XPS, PL, TPD and EPR are used to characterize the as-prepared catalysts. The results show that the charge transfer between V-N-g-C3N4 and Ag2CO3 follows the "Z-scheme" mechanism. The atmosphere has a significant effect on the nitrogen photofixation performance of as-prepared catalyst using methanol as hole scavenger. Under the atmosphere of 50% O-2 and 50% N-2, the as-prepared N vacancy-doped g-C3N4/Ag2CO3 heterojunction catalyst displays the ammonium ion production rate of 11 mg.L-1.h(-1).g(cat) (-1), which is 2.4 times higher than that under pure nitrogen atmosphere. A "two-path" ammonia production mechanism is proposed in this work.