The invention provides a method for the observation, identification, and detection of blood cells, which comprises a label-free third harmonic generation (THG) tomography having a property of least injury. Submicron morphologies and granularities of blood cells can be revealed and reflected through this method. Leukocytes with different granularities can thus be identified from the intensity and distribution of third harmonic generation signals generated within cells. Furthermore, the method of the present invention is capable of performing a noninvasive sectioning microscopy image in vivo. Without cell and tissue damage, label-free third harmonic generation microscopy can real-time observe the morphology and dynamics of blood cells flowing in vessels or trafficking in tissues； Red blood cells and leukocytes have different morphology in blood flow and can thus be distinguished by in vivo third harmonic generation microscopy.
