The electron field emission performance of CNT doped with one Boron atom and with one adsorbed H2O molecular (BCNT+H2O) was investigated through the calculations of density functional theory (DFT) in this paper. The results indicated that there is an atomic scale region of electron congregation near the top of BCNT+H2O, and the electronic density of states (DOS) round the Fermi level is enhanced obviously. It is expected that BCNT+H2O be suitable for the field emission of electrons,judging from the calculation results of DOS, HOMO/LUMO and Mulliken charge population.