Abstract: Based on Mie theory, an optical system design method was established which can improve the measurement accuracy of the particle size analyzer. In the particle size analyzer's optical system design process, the diaphragm within the measuring range of the sample cell used a zoom method, it not only constrained the reference rays trace data for every field point at the ring detector to make the spot diagrams minimum, but also constrained the reference rays trace data for every field point of each zoom position at the ring detector to make the spot diagrams minimum. The optical systerm converges rays, which truly reflects the physical meaning of the particle size analyzer's measurement theory, improves the measurement accuracy.