One transmission-mode extended blue GaAs photocathode is prepared by MOCVD , whose integral sensitivity is 1980 A/lm. Its spectral curve is compared with the spectral curve of ITT photocathode for analyzing optical structure. The comparison indicates that the differences lie in the thickness and the Al mole value of the Ga1-xAlxAs window layer, electron diffusion length, and back-interface recombination velocity, which make the photocathode in this experiment inferior to that of ITT in extended blue performance. However our surface electron-escape probability and the thickness of the GaAs active layer are in accordance with those of ITT, which leads their difference in the long waveband part to be less than in the short one. In addition, our absorptivity in the whole response waveband is smaller than that of ITT photocathode, which leads the spectral response and integral sensitivity of the domestic transmission-mode extended blue GaAs photocathode to be inferior to the exotic one.