Abstract: The paper aims to determine the effect of the volume of air tube on pulsation amplitude measured on-line by air purge method for pulsed extraction column. It is shown that the relationship between real and apparent pulsation amplitudes remains linear when the volume ratio of the air tube to air probes is not greater than 4. At the same time, the shape of air tube in the bottom settler is in detail discussed for satisfying the demand in nuclear reprocessing. Based on above two conclusions, the design method of air tube in the bottom settler is given.