Abstract: On the basis of"three steps model"and "diffusion equation",the author discusses the dependence of quantum efficincies for GaAs/GaAlAs transmission mode photocathodes on some parameters such as electron diffusion length,interface recombination velocity and photocathode layer thickness,which are important for the fabrication of the photocathodes.Especially,the relation between the quantum efficiency under front illumination and that under back iliumination is also explored for the requirement of the present fabrication processes of the photocathodes.Hence,theoretical basis is founded for estimating the quantum efficiency under back illumination according to that under front illumination.