Negative-electron-affinity GaAs photocathodes offer high quantum efficiency, low dark current, low emittance, and strong potential for long-wavelength expansion. They have a wide range of applications in vacuum optoelectronic devices and electron sources. For the transmission-mode GaAs photocathode, achieving higher quantum efficiency while maintaining a thinner emission layer is a challenge. In this study, a GaAlAs window layer with a novel subwavelength-periodic light-trapping structure is proposed to enhance the broad-spectrum light absorption of a transmission-mode GaAs photocathode with a thin emission layer. The optical properties of the photocathode were investigated primarily through optical simulations using the finite-difference time-domain method, and structural design research was conducted. Compared with the classical planar GaAs photocathode, the photocathode incorporating the subwavelength light-trapping structure showed a significant improvement in light absorption. By simulating and optimizing the line width, diameter, height, and arrangement of a nanoarray with a period of 600 nm, the optimal subwavelength array structure was determined to be a cylindrical array with a line width of 440 nm, a height of 490 nm, and a 1/4-period staggered arrangement. Its absorptance in the wavelength range of 500–930 nm reached 84.91%. Compared with the traditional planar structure, the improvement in optical absorptance in the near-infrared region was particularly significant.