Benefiting from a high quantum efficiency, low thermal emittance, and large absorption coefficient, InxGa1−xAs is an excellent group III–V compound for negative electron affinity (NEA) photocathodes. As the emission layer, InxGa1−xAs, where x = 0.15, has the optimal performance for detection in the near-infrared (NIR) region. Herein, an NEA In0.15Ga0.85As photocathode with Al0.63Ga0.37As as the buffer layer is designed in the form of a transmission mode module. The electronic band structures and optical properties of In0.15Ga0.85As and Al0.63Ga0.37As are calculated based on density functional theory. The time response characteristics of the In0.15Ga0.85As photocathode have been fully investigated by changing the photoelectron diffusion coefficient, the interface recombination velocity, and the thickness of the emission layer. Our results demonstrate that the response time of the In0.15Ga0.85As photocathode can be reduced to 6.1 ps with an incident wavelength of 1064 nm. The quantum efficiency of the In0.15Ga0.85As photocathode is simulated by taking into account multilayer optical thin film theory. The results indicate that a high quantum efficiency can be obtained by parameter optimization of the emission layer. This paper provides significant theoretical support for the applications of semiconductor photocathodes in the near-infrared region, especially for the study of ultrafast responses in the photoemission process.