Abstract: In order to get optimization results of the emission current and the focal spot size of the micro X-ray tube, the geometric structure of the cathode emitter is simulated. Firstly, the relationship among the direct thermal cathode emitter, the emission current density and the geometry structure is derived theoretically. Secondly, the discretization of the finite integral algorithm in solving the numerical solution of the electric field distribution is discussed. Finally, the geometric model is built by using the CST particle Studio software, and the geometric structure of the cathode emitter is optimized. When the distance between the filament and the control pole is 0.4 mm, and at the position the aperture diameter of the control pole is 0.6 mm, smaller focal spot and higher charge density distribution are attainable. The emitter fabricated has a maximum emission current of 85 A.