To estimate the field enhancement factor of the gated nanowire, the image charge model of floating sphere between parallel gate and cathode plates is proposed. The field enhancement factor of the gated nanowire is expressed by β=1/2（3.5+L/r0+W）, where L and r0 are the length and tip radius of nanowire, respectively, and W is a function of the gate-hole radius R, gate-cathode distance d and the geometrical parameters of the nanowire. The calculation results show that the influence of the aspect ratio of the nanowire on the enhancement factor is remarkable, i.e., the enhancement factor increases rapidly with the increase of the length and top curvature of the nanowire. Furthermore, the enhancement factor decreases with the increase of the gate-cathode distance and is equal to β0=3.5+L/r0 when the gate-cathode distance tends to infinite. The smaller the gate-hole radius, the larger the enhancement factor, and the enhancement factor will be equal to β=β0+1.202（L/d）3 when the gate-hole radius tends to zero.