The absorbing and scattering of laser beam by air molecules and fog particles were studied. Kruse model,single scattering approximation and Monte-Carlo simulation were adopted to calculate the extinction characteristics of three types of fog irradiated by near infrared laser of 0.86 μm wavelength,the results show that Kruse model is not applicable to calculate the extinction coefficient when the visibility is lower than 0.5 km. The extinction coefficient from the single scattering approximation with lower precition is larger than that from the Monte-Carlo simulation in the same visibility,which means a lower transmission at the same distance. The difference of extinction characteristic of three types of fog is so small in the same visibility that the influence of the size distribution of fog particle on it is not obvious.