Abstract: Since the proximity distance, especially the 1st proximity distance (d1) is the most important factor to influence the resolution, a new on-line testing method of proximity distance for the image intensifier is put forward. According to the principle of the parallel plate capacitor, the 1st proximity distance can be obtained by measuring the capacitance value C1 between the cathode surface and the input side of microchannel plate. The functional relation between C1 and d1 was fitted by the tested data. The function formula was modified and verified by the accuracy analysis. It is shown that the maximum error is 11.9% and the tested results meet the precision requirement. The process of press seal and the proximity distance of an image intensifier can be controlled precisely by this method. Therefore, the resolution is increased. The method is simple and could be used in other online testing of short distance.