The potential and the electric field at the end of the individual nanowire under an electric field were calculated with the image charge model. With the nanowire as a field emitter, the enhancement factor was given by the expression: β０＝h/ρ+35. Taking into account the influence of the anode_cathode distance, the enhancement factor at the end of the nanowire could be adjusted based on the equation: β=h/ρ+35+A(h/d)3, where h and ρ are the length and the radius of the nanowire respectively; d is the anode_cathode distance and A is a constant. From the above results, it is concluded that the aspect ratio is the most important factor for the nanowire emitter and the anode_cathode distance also slightly influence the enhancement factor of the nanowire in field emission, and the enhancement factor increases with the decrease of the anode_cathode distance.