The invention provides a two-free-curved-surface optical lens used for an ultrathin direct-lit type LED backlight system. The two-free-curved-surface optical lens comprises two free curved surfaces which serve as an incidence face and an emergent face respectively. According to the shape of the free curved surface lens, a spatial right-angle coordinate system is established with an LED light source as an original point, the plane where the LED bottom face is located serves as a plane XOY, the axis passing through the original point and perpendicular to the plane XOY serves as a Z axis, the light source three-dimensional angle is divided first, annular belt division is conducted on the illumination area on the target illumination face, the mapping relation between the light source three-dimensional angle and the annular belt area on the target illumination face is established according to the law of conservation of energy, and the final double-free-curved-surface lens is obtained through the geometric relation according to the laws of refraction and reflection. The two-free-curved-surface optical lens is small in size, light rays emitted by LEDs can form circular light spots which are uniform in luminance distribution after passing through the two-free-curved-surface optical lens, and the output efficiency of the lens is quite high.