The invention relates to an side light type LED backlight, which comprises a LED lamp set, a light guide plate, an optical film group, wherein the light leaving surface of the LED lamp set, the incident plane of the optical film group are in a closed space; the space, in which the LED lamp set except the light leaving surface is arranged, is communicated with the exterior space of the side light type LED backlight. In the proposal, the closed space of the side light type LED backlight can avoid light-leakage, and because the space, in which the LED lamp set except the light leaving surface is arranged, is communicated with the exterior space of the side light type LED backlight, energy emitted by a Led lamp set base can be quickly transmitted to the exterior space, thereby improving heat dissipation performance of the side light type LED backlight and avoiding light- leakage furthest.