In a rectangular light emitting diode (LED) array, by symmetry analysis of illuminance, a segmented illuminance profile of a light unit is presented to build free-form lenses for direct panel lights′ uniform illuminance. Each light unit is composed of an LED element and a rotationally symmetrical free-form lens, whose refraction index is 1.4935. A square array of LED direct panel light with a distance-height ratio of 4.1 is given as a goal, then the freeform lenses are built by energy distribution design methodology, and the optical simulation results show that the uniformity on the target screen is greater than 0.85, and the optical effect is higher than 0.9. This method can be applied to the extend light sources. The limit of illuminance uniformity of the direct LED panel lights mentioned above can be achieved by using the symmetrical analysis method which is useful for the optical design of LED panel lights.