The first-principles density-functional theoretical calculations are performed to investigate the effects of N doped and N, M (Cd, Mg) codoped on the geometrical structures and field emission properties of capped (9, 0) zinc oxide nanotubes (ZnONT). The results show that the N could improve the stability of the structure of capped side. With the increase of the applied electric field, the density of states (DOS) shifts towards the low energy position, the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and the effective work function decrease drastically, and the electrons congregate to the capped side. The analyses of DOS/local DOS, HOMO/LUMO, and Mulliken population indicate that the field emission properties of N, Cd-Codoped ZnONT are improved, but those of N, Mg-codoped ZnONT are worsened.