Water fogs system can mightily attenuate the infrared spectrum, so more and more attention has been paid to this problem in military. Based on the Mie theory, the collective characteristic of the particles forward scattering was analyzed. The near forward scattering ratio was defined, also the particle size and scattering DBC case were confirmed when the Lambert-Beer'law was applied. A mass of calculation revealed that the visual extinction coefficient calculated with the combined parameter p=x·θ was not accurate, and that the near forward scattering ratio was in direct proportion to both the particle size and scattering angle in the band of middle and far infrared. When the infrared wavelength was fixed, the near forward scattering ratio was in direct proportion to the particle radius. Finally, according to the law of forward scattering, two experiential functions whose variables were not only the p=x·θ were given, by which the correction calculation for light extinction was made easy and exact.