The invention relates to a preparing method of thermo-electron source, which comprises the following steps: providing a base plate and forming a first electrode and a second electrode at interval on the surface of the base plate, and forming a one-carbon nanotube membrane structure to cover the first electrode and the second electrode to be a thermo-electron emitter. At least portions of the one-carbon nanotube membrane structure pass through the gap arranged between the first electrode and the second electrode and the base plate, thereby achieving a thermo-electron source.