Experimental and theoretical analysis results show that there is an optimal Cs/O ratio for GaAs photocathode to activate successfully. In order to change Cs/O ratio of GaAs photocathode,the method applied is to regulate theheat currents of Cs resource and O resource in experiment. Activation experiment results show that the samples activated on appropriate Cs/O current ratio can achieve the optimal results,their photocurrent ascent rate is the largest during the first addition of O,quantum efficiency is the highest and stability is also very well. When deviation the appropriate Cs/O current ratio,activation experiment results are not as good as the former. Along with the increase of Cs/O current ratio,the total times of (Cs,O) alteration decrease. The Cs/O current ratio during the first (Cs,O) alternation should be regulated well,the Cs/O current ratio which can achieve the largest ascent rate is the optimal Cs/O current ratio. When regulated well the ratio should be maintained during activation process.