The invention discloses a small-sized ion source and a preparation method thereof, and belongs to the technical field of ion generation of field emission electron ionization gas. The problems are solved that existing ion sources are complex in structure, large in size, high in cost and inconvenient to use, the magnitude of current of an ion beam generated by the existing ion sources is very difficult to control, and real-time adjustment cannot be achieved. The small-sized ion source comprises an insulating sleeve, electron guns encapsulated at both ends of the insulating sleeve and used for generating high-energy electrons, second grid electrodes, focusing electrodes, extraction electrodes for extracting an ion beam and the like; gas inlets are formed in the portions, at the ends of the electron guns, of the insulating sleeve; the electron guns include insulating bases, field electron emission electrodes which are encapsulated on the insulating bases and connected with the ground or negative voltage and first grid electrodes connected with positive high voltage. The small-sized ion source is used for generating the ion beam.