The difference between the diffusion drift length of photoelectrons in exponential-doping GaAs photocathode and that in uniform-doping GaAs photocathode is studied. According to quantum equations, the optimized thickness of transmission-mode exponential-doping GaAs photocathode is simulated to be 20 μm. Two transmission-mode exponential-doping GaAs samples with the thickness of 16 and 20 μm are activated by (Cs,O) alternation technique. Integral sensitivities of the two samples are 1228 and 1547 μA/lm, respectively. The ratio of integral sensitivities of the two samples is 0796∶1, which agrees with the simulation result.