Abstract: The rod-pinch diode (RPD) is a cylindrical pinched beam diode being developed as an intense pulsed small diameter bremsstrahlung source for flash X-ray radiography.The studying significance of the physical characteristics lies in optimizing geometry structure of RPD.Using PIC program and numerical simulation, a model of the rod-pinch diode operation based on space limited current at low voltage and self-magnetically limited current at high voltage is researched.Analysis shows that the diode characteristic is determined by space charge limited current at low voltage and self-magnetically limited critical current at high voltage.The simulation result is consistent with reference.Under the special conditions (space charge limited, weakly pinched and self-magnetically limited), the ratios of ion current to electron current are studied.The coefficient for calculating the self-magnetically limited ion to electron current ratio in the self-magnetically limited condition is affected by the electron and ion transit time and free from the high voltage.