Abstract: Based on the classical theoretical formula of one-dimensional space charge limited current for coaxial diode, the change regulation of two-dimensional space charge limited current of the hot electron outwardemission coaxial diode with the geometric parameter of the diode and the influence of geometric structure on the critical condition of virtual cathode generation were studied by CST particle studio. The results show that the correction coefficient of diode is a monotonic decreasing function of aspect ratio. The correction coefficient decreases gradually as the ratio of anode radius to cathode radius increases. The voltage of diode, the radius of anode and cathode and the length of cathode all affect the production of virtual cathode. The study result in this paper will provide reference and theoretical basis for the design and optimization of RF tetrode structure.