To obtain a high resolution of the reflection-mode AlGaN photocathode by establishing the modulation transfer function (MTF) model of this photocathode, the influence of emission layer thickness Te, electron diffusion length Ld, recombination velocity at back-interface Vb, and optical absorption coefficient α on MTF for varied-doping and uniform-doping Al0.42Ga0.58N photocathodes have been given. The computational results suggest that varied-doping structure has great potentiality in improving both resolution and quantum efficiency of the reflection-mode Al0.42Ga0.58N photocathode. This improvement is mainly attributed to the reduced lateral diffusion of photoelectrons, which is caused by an electric field generated by the varied-doping structure, and hence the photoelectron transportation towards photocathode surface is promoted.