In order to improve the classification accuracy of cell subsets, a telecentric system which is based on the characteristics of cell’s scattering is designed. The irradiating and receiving light work collinearly in this system, and the optical receiving efficiency of back scatter light increases to twice the origin by using a special mirror. The optical design parameters and structures are given by using aberration theories and optical design software. The performance and signal collection of the optical system are analysised. When the resolution is 100lp/mm, the MTF is more than 0.17. The stray light is effectively filtered out and the accuracy of scattering signal is improved. The optical system can be used to identify eukocyte.