A large area cold cathode based on carbon nanotube films has been successfully fabricated. Investigation of the explosive field emission properties of the carbon nanotube cathode in a double-pulse mode was carried out and a high emission current density of 245 A/cm2 was obtained. The turn-on time of the CNT cathode is about 40 ns. The time- and space-resolution of the electron beam emitted from the cathode was investigated. The formation of the cathode plasma layer was proved and the plasma expanded at a velocity of ～8.17 cm/μs toward the anode. The plasma has big influences on the intensity and distribution of electron beams. The formation of cathode plasma had no preferential position and the local enhancement of electron beams was random. This carbon nanotube cathode appears to be suitable for high-power microwave device applications.