The first-principles calculation was performed to study the electron field emission properties of carbon nanocones （CNC） with different cone angles and geometrical structures. The results show that with the increase of the applied electric field （Eadd）, the electronic structure of CNC changes significantly, the density of states （DOS） shifts towards to low energy position. Especially, DOS at Ef increases and the pseudogap decreases respectively, and a majority of electrons congregate at the close end. The investigation of DOS, HOMO/LUMO and Mulliken population analysis indicates that CNC3 and CNC4 are very suited for electron field emission.