The modulation transfer function （MTF） of transmission-mode exponential-doping photocathodes has been solved from the 2-dimensional continuity equations. According to the MTF equation，we calculated and analyzed the theoretical resolution characteristic of the exponential-doping photocathodes. The calculated results show that，compared with the uniform-doping photocathodes，the exponential-doping structure can increase the resolution of photocathodes evidently. For the spatial frequency franges between 100 and 400 lp/mm，the increase of resolution is more pronounced. Let f=200 lp/mm，the resolution of exponential-doping photocathodes generally increases by 20%—50%. The resolution improvement of exponential-doping photocathodes，as well as the quantum efficiency improvement，is also attributed to the built-in electric field.