Abstract: In order to detect the helium density in thermosphere with a laser radar,after theoretical analysis and computer simulation,the atmospheric absorption coefficients for the three resonance lines making up metastable He 1083nm emission line were calculated,and the atmospheric absorption coefficient was found at the order of magnitude of 10-3.The results show that for the same spectral line,the atmospheric absorption coefficient is proportional to the concentrate of atoms and the fourth root of the temperature,however it is inversely proportional to the pressure.For different spectral lines,the atmospheric absorption coefficients are proportional to the central wave length.In the half width of the spectral line,the average atmospheric absorption coefficient is proportional to the atmospheric absorption coefficient at the line center.Finally,the application of atmospheric absorption coefficient in spectrum analysis was pointed out.This calculation has some reference significance to realize the detection of the helium density in thermosphere.