The rod pinch diode is perfect as a source of accelerators for flash X-ray radiography by virtue of a small and stable spot. But it is not suitable for intensive current drivers because of high diode impendence of 40∼60 Ω. However, by employing pre-filled plasma into diode prior to the driving current, the diode impendence can be effectively reduced. Plasma density plays an important role in this process, especially for sheath formation and space charge current in the diode. Analysis and simulation results show that a proper range of plasma density could be 10 15 ∼10 16 cm −3 .