According to the morphological structures of leukocyte, the double layer eccentric-sphere model is established for a kind of single-nuclear cell. Based on the Mie scattering theory and interface transfer theory, the expressions of amplitude functions are revised by the geometrical-optics approximation. Results of numerical calculation are consistent with the results from the VirtualLab imitation system which calculate by finite element method. Results of numerical calculation also show that three kinds of intensity modulations with different frequencies can be found in the angular distribution. The physical mechanism about these three modulations has been analyzed. The relations between the modulation characteristics and the physical and optical parameters of the cell are obtained. The modulation distortion phenomenon will appear in the case of nuclei close to the cell. These provide a useful theory foundation for improving the measuring and identification technique of single cells.