On the basis of the properties of two beams in heterodyne detection, the photon statistical models for the coherent light and multi-mode thermal light fields were analyzed. The second and third order statistical characters of the two kinds of optical fields were discussed, in which the second order statistics was corresponding to the Fano factor, while the third order one was to the symmetry parameter. The Fano factor and the symmetry parameter for the two kinds of light fields were measured experimentally. The results indicate that Fano factor and symmetry parameter do not change with the variance of incident photons for the coherent light field. Combined with the theoretical model and experimental data, it shows that the cross-talk probability is 0.032 1 and the gain coefficient is 1.0460 ph. However, the Fano factor of multi-mode thermal light field increases with the incident photons and gives a linear ascending trend. The factor reduces with the increase of the target velocity, which means the fluctuation of the echo photons decreases because of the averaging effect caused by increasing the number of thermal modes. Moreover, the symmetry parameter of multi-mode thermal light field is parabolic curve distribution. The research of coherent and multi-mode thermal light statistics provides a foundation for the laser heterodyne detection based on photon counting.