Noble-metal-free catalysts are highly desirable for hydrogen generation from formic acid dehydrogenation. Herein, using first-principles density functional theory calculations, we design a series of nickel-anchored nitrogenated holey two-dimensional carbon structures (Ni-x@C2N, x = 1-3) as formic acid dehydrogenation catalysts. For all Ni-x@C2N surfaces, the formic acid dehydrogenation preferably proceeds via the formate pathway. The effective barrier continuously decreases for formic acid dehydrogenation while increases for hydrogen formation from Ni-1@C2N to Ni-3@C2N. The side reaction producing carbon monoxide and water via the carboxyl or formyl pathway cannot occur on Ni-1@C2N or Ni-2@C2N and is not preferred on Ni-3@C2N, and thus, the Ni-x@C2N catalysts possess excellent selectivity of hydrogen. Notably, the unsaturated nitrogen atom of substrate also participates in the reaction and exhibits synergetic effect with the nickel component in Ni-1@C2N and Ni-2@C2N. The Gibbs free energetic span analysis predicts that the order of reactivity is Ni-2@C2N (0.79 eV) > Ni-1@C2N (0.87 eV) > Ni-3@C2N (1.23 eV), and the turnover frequency of Ni-x@C2N is evaluated. The results are compared with the experimental and theoretical reports of some palladium-based catalysts. The present work suggests that the Ni-x@C2N may be promising noble-metal-free catalysts for formic acid dehydrogenation with high performance and low cost.