Abstract: Pulse emission current at the inflection point on I-U curve of a thermionie eathode can be measured rapidly by means of a circuital technique. This method can prevent the eathode from suffering injury during the measurement. So that it can be used to compare the emission levels between different thermionie cathodes, and to monitor the changes of activation of cathodes during life test. The principle of the rapid-measurement of pulse emission current at the inflection point on I-U curve of the thermionic caathode is discribed. Autodisplay of the inflection point on the I-U characteristics is realized. The method of verification of its correctness is given. A great deal of measurements on various thermionic cathodes show that the instrument is of practical value. Finally the measuring error of the instrument is discussed.