The electron field emission performances of BN nano tube （BNNT） capped and doped with one carbon atom are investigated through the first principles calculations. The results show that the electronic structures of the systems change obviously. The shift of margin of the density of states （DOS） towards low energy position increases as the applied electric field and the highest occupied molecular orbital （HOMO）-lowest unoccupied molecular orbital （LUMO） gap decrease. The analyses of DOS, local DOS，HOMO, LUMO and their gap consistently indicate that CeqBNNT system is more propitious to the electron field emission than other systems.