Continuous spectra transmittance is measured and studied from visible to near infrared bandsusing sun photometer. The atmospheric transmittance is obtained with the solar spectra measured by the accurate calibration sun photometer by Langley method to calculate the irradiance arrive at the top of the atmosphere. From the analyses of different time,aerosol contents,seasons and models,the characters of the atmospheric transmittance are achieved in the sunny,cloud free atmospheric condition. The results can provide the base for target detection out of the atmosphere.