This study estimates photosynthetic active radiation（PAR） based on a simplification of the general radiative transfer equations and Moderate Resolution Imaging Spectroradiometer（MODIS） atmosphere products（aerosol

cloud

ozone and water vapor product）.Comparing estimated PAR with field measurements in Tibetan Plateau in 2005

significant linear correlation in order 0.8—0.9 is obtained and relative error is less than 13%.Remaining other condition constant in sensitive analysis

cloud thickness

depth of aerosol and pressure show more important impacts on PAR estimation than water vapor and ozone.Cloud thickness

angstrom turbidity coefficient

atmospheric pressure changes by 30%

25% and 15% will lead to respective changes in PAR of 20%

5% and 5%.