Traditional five classification hematology analyzers usually improve white blood cell (WBC) classification accuracy by combining chemical methods, which will increase the complexity of instrument structure and the cost of use and repair. An optical system with a special structure is designed, which introduces the detection of back scattered light by the means of combining direct current (DC) signal, laser forward scattering signal and back scattering signal to achieve the WBC five classification. The experiment results show that the relative deviation of the prototype with the sensing system for LYM, MON, NEU, EOS and BAS is less than 5%, and its performance is close to the hematology analyzer Mythic 22. The optical system after optimization can complete in situ detection and classification of WBC and has the characteristics of simple structure and stable performance.