Abstract: The extinction characteristics of aerosol have great influence on the effective transmission of laser propagating in atmosphere. In recent years, with the declining of air quality, the aerosol pollution is becoming serious. In order to analyze the dependence of extinction coefficient on ambient relative humidity(RH) under two cases of weather in coastal area, visibility, as well as relative humidity in the southeast coastal area of China is measured by experiments. The results show that the deliquescence points of aerosol particles under the two kinds of weather are both about 80%, and that after deliquescence, the extinction coefficient increases with the increasing of relative humidity. For the laser with a wavelength of 1.06 m, an order of difference of magnitude for extinction coefficient of haze and fine weather is obtained. However, when 80%＜RH＜90%, relative increasing of the aerosol extinction coefficient under two conditions is similar, the f(RH) ranging from 1 to 2 mostly; for RH＞90%, the difference is very large. The f(RH) increases by two orders of magnitude in good weather, which is of significance on laser propagation in the atmosphere.