The model of floated sphere in the triode configuration was proposed and the actual electric field and the field enhancement factor at the top of carbon nanotube were calculated analytically by superposition principle of electric field. The effects of the contact resistance, the geometrical parameters of device, and the dielectric of between the gate and cathode were computed. The calculation results show that the emission current from carbon nanotube is greatly reduced by the contact resistance. When the contact resistance is larger than 100 kΩ, the emission current from carbon nanotube tends to be zero. The field enhancement factor was at a maximum value under the condition of the vacuum. The gate-hole radius, the gate-cathode distance, and the gate-anode distance greatly affect the field emission properties of the device. The field emission properties improved via optimizing the parameters above mentioned and modulating the gate voltage.