In free space optical communication system,the characteristics of atmosphere are very important. These effects include absorption and dispersion of aerosol,dust,frog,rain and haze when laser beam propagates through atmosphere. The laser power attenuation by the absorption and dispersion is introduced. Then the turbulence and its influence of flicker,bend and abruption are analyzed. At last the experimental system and the test method are given. The experiments are carried out in the different air condition at the same time in a day,in different time at the same day and in different visibility condition. The results are in accordance with theoretical analysis.