A method for making a thermionic electron emission device. The method includes the following steps. First, an insulating substrate is provided. Second, a number of lattices are formed on the insulating substrate. Third, a first electrode and a second electrode are fabricated in each lattice on the insulating substrate. Fourth, a carbon nanotube film structure is provided and at least part of the carbon nanotube film is suspended structure above the insulating substrate. Sixth, excess carbon nanotube film structure is cut away to obtain a number of thermionic electron emitters. The thermionic electron emitters are spaced from each other and located between the first electrode and the second electrode in each lattice.