The gallium arsenide (GaAs) photocathode was generally cleaned by radiant heating, direct heating, ion bombardment annealing, and so on. In this paper, the radiant heating method, namely thermal cleaning method, was adopted for GaAs photocathode surface purification. Using this method could obtain an atomic clean surface, ensure the integrity of the GaAs surface lattice, and guarantee the uniformity of surface cleaning effect at the same time. But because the accurate measurement of the GaAs photocathode surface temperature in the vacuum system was very difficult, the residual gas analyzer (RGA) was used in this experiment to monitor the residual gas composition in ultrahigh vacuum during the thermal cleaning process and determine the thermal cleaning temperature by the partial pressure curves of As and Ga. It was found that the first peaks of As and Ga elements both appeared after heating about one hour, accompanied with H2O, N2/CO, CO2 and other common gas. According to partial pressure curves of H2O, N2/CO, CO2 and the heating time, it could be judged that the temperature at that time was not high, which should be under 150°C.After thermal cleaning experiment of three GaAs photocathodes, it was found that the peak value of As partial pressure at low temperature was generally within 10-11mbar~10-10mbar, and the peak value was at 10-10mbar at high temperature. Sometimes it was appeared that the peak value of As partial pressure at low temperature was even higher than the peak value at high temperature. The As volatilization phenomenon occurred at low temperature indicated that the elemental As exist on the GaAs photocathode surface or near surface after the chemical etching process, and the As could volatilize from GaAs photocathode at low temperature in the beginning of thermal cleaning. This research has guiding significance for further understanding the thermal cleaning mechanism of GaAs photocathode and improving the thermal cleaning technology.