With the continuous development of the automobile industry, head-up display (HUD) technology has been widely used in modern automobiles. In the Picture Generation Unit (PGU) of a HUD system, since the liquid crystal itself does not emit light, the backlight module is required to provide lighting. The illumination uniformity of the LED array light source in the backlight module is crucial to the performance and display effect of the backlight module. In order to improve the illumination uniformity of the backlight LED array, this study optimizes the LED array arrangement using the simulated annealing algorithm to achieve uniform irradiance on the receiving plane. First, the irradiance distribution function of the LED array is derived, and an evaluation function is constructed to measure the uniformity of the illumination. The simulated annealing algorithm is applied to calculate the optimal LED array spacing, and the geometric model is imported into optical simulation software for validation. The simulation results demonstrate that the algorithm effectively improves the uniformity of irradiance. Furthermore, the introduction of a diffuser film further improves this uniformity.