The relationship of polarizability and scattering number has great value to extract the backscattering light deeply into the tissue. The relationship of polarizability of transmitted light and the scattering number of light in medium is analyzed. The scattering number that purely depolarization needed is given. And the result is validated by directly scattering experiment. The conclusion can be made that when the scattering number is 14, the polarization light will be depolarized.