Abstract: The surface of the active layer of GaAs photocathode must reach the atomically clean surface before Cs-O activeness.The most effective method,which is used quite often,is the hight-temperature thermal cleaning one.It is very difficult to measure precisely the surface temperature of a photocathode in a vacuum system during the thermal cleaning. This paper discusses how to use a quadrupole mass spectroscope to analyzed the whole process of thermal cleaning before the activeness of GaAs photocathode,thus the temperature and technique of the optimal thermacleaning is determine.The problem for the thermal cleaning technique taken before GaAs photocathode activeness is well rolved.