Frustrated Lewis pairs (FLPs), new catalysts with pairs of spatially frustrated Lewis acid and Lewis bases within or between molecules, have aroused high attention due to their strong activation ability for inert small molecules such as CO2 and N2. Herein, Mo is introduced as a Lewis acid site to endow porous ultrathin carbon nitride (g‐C3N4) with FLPs‐like properties. The as‐prepared Mo‐doped porous ultrathin carbon nitride (Mo(0.005)–CN) exhibits N2 fixation rate as high as 479 μmol g−1 h−1, which is seven times more than that of bulk g‐C3N4. This is mainly attributed to the synergistic effect of two aspects. First, Mo(0.005)–CN has an excellent light utilization ability. Second, the charge transfer efficiency between the d orbital electrons of metallic Mo and the g‐C3N4 energy band is enhanced in Mo(0.005)–CN. This work paves the way for designing g‐C3N4 from the perspective of FLPs for high photocatalytic N2 fixation efficiency.