Conventional backlight system of liguid crystal display (LCD) always uses side-in light emitting diodes (LEDs). It requires that the thickness of the light guide plate should be larger than LED′s light-emitting section width. To solve this problem, a free- form optical coupling module for side- LED backlight is proposed, which includes collimator lens and a free- form reflector. The light is firstly collected and collimated by immersing collimator lens, and then reflected and gathered into the LGP′s incident surface. By using Snell′s law, differential geometry method and aplanatic principle, the detailed design of the module is presented, and the feasibility is shown by optical simulation. Simulation results show that by choosing 0.4 mm×0.4 mm LED chip as the light source, the thickness of the light guide plate is reduced 0.6 mm, and the light utilization can still reach 96.23%. Compared with the conventional structure, the light utilization increases by 11.22%. The optical coupling module can realize a thinner backlight module effectively, and simultaneously help to restrain the hot phenomenon.