An optical film with microstructures for use in MiniLED backlight modules is designed to reduce the backlight module thickness. First， two diffusion principles based on the diffusion particle and refraction lens are analyzed. It is found that when the optical distance （OD） is very small， none of the principles are effective in ensuring that the light is fully diffused in order to meet the requirement on uniformity. Then， based on the principle of total reflection， the surface source is discreted to a cluster of point sources， the energy returned by a single point source to the lamp board is analyzed， and the energy returned by all the discrete point sources is accumulated. Notably， the profile of a microstructure can be decided when the energy returned by the surface source is maximum. The simulation results show that when the double-layer microstructure optical film is placed on the surface of a MiniLED chip， the brightness uniformity is 79.9%， which can be further increased to 89.2% by adding a diffusion film at an OD of 0.9 mm. Maskless direct writing lithography is used to prepare a sample optical film. The experimental results show that the brightness uniformity of the double-layer microstructure optical film placed on the surface of the MiniLED chip is 79.6%， which can be increased to 88.7% by adding a diffusion film at OD=0.9 mm. The optical film thus designed can be used to realize the ultrathin backlight module without the requirement of a precise position and therefore presents strong practicability.