It is difficult to achieve concurrent removal of nitrate (NO3- -N) and ammonia (NH4+ -N) from water thoroughly by photocatalysis due to their opposite charges and inappropriate potentials of photo-induced carriers. Herein, an innovative magnetic field enhanced 3D/2D Mn2O3/g-C3N4 photocatalytic system was developed for denitrification from NO3- -N and NH4+ -N contaminated water under visible light. High removal efficiency of 94.5% and 97.4% for NO3- -N and NH4+ -N were achieved after imposing magnetic field. Impressively, the extra magnetic field integrated the advantages of providing driving force for NO3- -N and NH4+ -N approaching catalyst surface as well as facilitating separation of photo-induced charge carriers in newly-formed II type heterojunction by generating Lorentz force. Interestingly, the electrons on conduction band of g-C3N4 could reduce NO3- -N to NO2 -N, N-2 and NH4+ -N. Subsequently, the produced NO2- -N could be reduced to N-2 and NH4+-N. Meanwhile, the holes on valence band of Mn2O3 could convert NH4+ -N to N-2 selectively, which made the dominate contribution to high N-2 recovery (93.2%) during photocatalysis. Moreover, the reusable Mn2O3/gC(3)N(4) achieved efficient denitrification under magnetic assisted photocatalysis in real water backgrounds such as tap water and river water. This study provided an efficient and promising strategy for nitrogen remediation from water.