Abstract: The forming and drifting of plasma in the intense electron beam diode is one of the important issues in studying the diode's operation. In this paper, based on the Child-Langmuir law and conduction coefficient of the cool-cathode diode, it is suppose that the cathode emission area of intense electron beam diode is variable. The drift velocity of the plasma in the diode is studied with experimental data of FLASH-Ⅱ accelerator. The result shows that the drift velocity of plasma is variable with time. The qualitative analysis about this velocity presents the closing speed of the gap between the diode's two poles and also fully explains the plasma's movement while the diode is on. 