A radiation source of a near-end treatment device comprises a thermionic cathode being heated to generate free electrons, wherein the free electrons are emitted to a target element to form an electron beam under the action of an accelerating field; a focusing element, wherein the accelerating field is partially around the thermionic cathode and has a focusing electric field, and the electron beam shortens the diameter of the electron beam through the focusing electric field; an anode tubular column configured to enter a lesion position or the inner cavity of an applicator; the target element located at the tail end of the anode tubular column and bumped by the electron beam to generate radiation; and a control module configured to control the focusing electric field of the focusing element so as to regulate the diameter of the electron beam.