The direct-lit backlight used in an automobile head-up display must have high brightness and a small size. To reduce the thickness of a direct-lit backlight and improve its brightness, an optical brightness enhancement film is designed. First, a two-dimensional microstructure curve for the uniform illumination of light emitted by a single light-emitting diode (LED) is designed according to Snell's law. A microstructure curve is designed and calculated with an LED as an ideal point light source. Based on the consideration that the actual LED is a square luminous surface, the microstructure curve is optimized and its uniform illumination function to a real LED is improved. Then, based on the arrangement of LEDs, a hexagonal honeycomb arrangement is proposed and a simulation analysis is conducted. Simulation results show that the central brightness of use of the optical brightness enhancement film is increased by 173.2%. From the results of the microstructural design, a practical sample is produced by using the maskless direct lithography process and its effect is tested. Test results show that the central brightness is increased by 136.2% and the thickness is reduced by 13 mm. The proposed optical brightness enhancement film can effectively improve brightness and reduce the thickness of backlight units.