The invention belongs to the field of optical lenses, and particularly relates to a one-step packaging LED (Light Emitting Diode) lens with a free curve and a design method of the same. The lens comprises a plane and the free curve, wherein the plane corresponds to an inner surface, and the free curve corresponds to an outer surface and is used for realizing uniform illumination and rectangular light distribution. When a light emitting surface of an LED chip emits light beams, the light beams are received by a primary lens after being refracted through the inner surface, and are emitted on a road surface through the free curve. The shape of the free curve is obtained through numerical solution based on the energy conservation law and the requirements for the illumination uniformity. As the one-step packaging lens comprises the free curve, all light emitted by LEDs can illuminate the road surface; thus, the utilization rate of the light beams is increased, and the 'zebra pattern effect' is avoided. When the lens is used, the uniform illumination can be realized through combining a plurality of packaged LEDs, the link of the secondary optical design is eliminated, and therefore, a lot of time is saved. The one-step LED light distributing lens is widely used, is suitable for all fields in need of uniform illumination, and has great social and economic benefits.