A compact power spectrum collection lens(PSCL) system for the laser diffraction particle size analyzer was proposed.In this system the former lens component is a short focal length,a magnifying lens of 4 times is as the later lens component to change focus length of the system from F500 to F4000.The length of the optical system is no more than 1 020 mm.The designed results show that the method makes the length of the whole system shorter,and has good image quality.