Abstract: The time response characterisitics of In0.53Ga0.47As/InP heterojunction infrared photocathode was studyed in this paper, such photocathode worked at transmission mode with a wide spectral response range from 1.0-1.7 m. Under certain field-assisted bias voltage, the transmission characteristics of photo-excited electrons inside the phococathode were simulated. The results show that the response speed of the photocathode are accelerated with the increasing of the field-assisted bias voltage. While P-InGaAs photo-absorbing layer is thickened, the response speed gets slow. The response speed also gets slow when increasing the doping concentration of InGaAs photo-absorbing layer. When increasing the thickness and doping concentration of P-InP photoelectron-emitting layer, the respose time will be prolonged. Based on all these conclusions, the structure parameters and doping concentration of each layer were optimized. The optimization results show that when the thickness of the photo-absorbing layer and the photoelectron-emitting layer are about 2 m and 0.5 m respectively, and the doping concentration of photo-absorbing layer and photoelectron-emitting layer are about 1.51015 cm-3 and 1.01016 cm-3 respectively, the response time of photocathode can be reduced to less than 100 ps.