Abstract: In an accident of operation of JJ-2 electron Van de Graff accelerator, anoteworthy phenomenon was found, i. e., when the accelerator was shut downand its high vacuum pump was withdrew, a phenomenon was found that theradiation field in great strength was appear under the window of electron outletand its surroundings. After the accident has happened, a series of simulatingexperiments were carried out, and the phenomenon was confirmed. This papertries to interpret the phenomenon.