An algorithm used to design a free-form lens for the extended light-emitting diode (LED) source is proposed. The design of a free-form lens is realized by the overlap of the weighted lens profiles of several point sources and the evaluation function reflecting the illumination uniformity is constructed, in which the independent variables are three weighting factors. The particle swarm algorithm is used to optimize the weight factors and minimize the evaluation function, and thus the illumination uniformity is the highest. Correspondingly, the illumination uniformity on the target surface increases from 78.93% for the initial structure to 90.67% and simultaneously the light efficiency is kept above 92%. The fabrication error analysis and lateral tolerance analysis of lens show that the influence on the illumination uniformity on the target surface is small when the fabrication error is within the range of -1.0-1.0 μm and the lateral displacement error is within the range of 0.0-0.2 mm.