Abstract: The gallium arsenide wafer was activated by excessive-oxygen YOYO activation after high-temperature cleaning on the wafer. The quantum efficiency and the lifetime under laser illuminating were measured, and the maximum quantum efficiency was 4.66%. An average current over 1 mA was obtained in the DC-gun experiment. In the end, we discussed the reasons for vacuum degree, high temperature cleaning and Cs-O activating process causing the low maximum quantum efficiency and low lifetime of the gallium arsenide wafer.