Compared with other cathodes, LaB₆ cathode has the advantage of high electron emission current density, and has been widely used in electric propulsion, high emission density electron gun and other products. In order to analyze the poisoning characteristics of the LaB₆ hollow cathode and identify its failure mode, the performance change features and reasons of LaB₆ hollow cathode exposed to atmosphere were studied. The results show that the LaB₆ hollow cathode adsorbs a great deal of poisonous gas on the surface of the emitter during exposure to the atmosphere, and while the work function from the emitter surface increases, the emitter appears short-term poisoning phenomenon. The discharge voltage and hollow cathode plate temperature are increased, and the discharge voltage and cathode top temperature are restored to the initial state after short-term operation. However, if the poisoning phenomenon of LaB₆ hollow cathode is serious, the oxide generated on the surface can not be removed by heating and ion bombardment. Therefore, the emitter state of hollow cathode can be indirectly characterized by monitoring the change of cathode temperature and anode voltage during the test.