The structural stability and electronic field emission properties of carbon nanotubes doped with a boron atom in different layers and adsorbed with several H2O molecules, as well as located in the applied electric field, are analyzed by means of the density functional theory based on the first-principles. The results show that the structure of B3CNT+5H2O doped by a boron atom in the third layer and adsorbed with five H2O molecules is most stable, the distribution of Mulliken charge on the tube cap is most dense. In particular, compared with the B3CNT doped by a boron atom and CNT+5H2O adsorbed with five H2O molecules solely, the density of states at the Fermi energy level for B3CNT+5H2O increases by 20% and 33% respectively. Therefore, the latter has the best field emission property.