The present invention mainly relates to the structure and manufacturing process of a new f.theta. lens in LSU, which is plastic lens produced through injection molding. Its optical surface is made of multi-sections, and the optical surface of each section, according to the angles formed between the sections and the laser beams ripping-in, are designed with a specified coefficient set. Therefore, this new f.theta. lens, with high tolerance in assembling and high performance in scanning, avoids the demerit of conventional f.theta. lens, each optical surface of which has only one coefficient set. According to the curve fall formed at the border of every two optical surfaces, we find out a way to ascertain the best continuing surface profile, which is curve fitting simulation and optical simulation. At last, we produce a mold for the f.theta. lens with multi-sections optical surface by using super-precise machine process to control the mold processing and use the mold for injection molding, and then manufacture lots of f.theta. lens with multi-sections optical surface.