To explore the structural feature of high performance transmission-mode GaAs photocathode, the optical properties and shortwave limitation for the transmission-mode quantum efficient formula is modified. By using the modified formula, a high quantum efficient (43%) curve of ITT is well fitted. A series of structural parameters is obtained with in a relative error less than 5%, which indicates that the thickness of the Ga1-xAlxAs window layer is 0.30.5 m, the Al mole value is 0.7, and the thickness of the GaAs active layer is 1.11.4 m. In addition, an optimized structure for the uniform-doping transmission-mode GaAs photocathode is suggested based on the fitted results. When the thickness of the Ga1-xAlxAs (x=0.7) layer and the GaAs layer are 0.4 m and 1.11.5 m respectively, the integral sensitivity can exceed 2350 A/lm.