The electron field emission properties of C-doped capped single-walled BNNT （C@BNNT） are investigated by first-principles study. The results show that with the increase of the applied electric field, the electronic structure of C@BNNT changes significantly，the density of states （DOS） shifts towards the low energy position，the local density of states （LDOS） at the Fermi level increases dramatically，the energy gap between the lowest unoccupied molecular orbital （LUMO） and the highest occupied molecular orbital （HOMO） decreases drastically and the electrons congregate to the capped side. The investigations of DOS，HOMO/LUMO and Mulliken population analysis indicate that， compared with pristine BNNT, the field emission properties of C@BNNT, especially of C@BNmoreNT， are greatly improved.