Photosynthetically Active Radiation（PAR） is an important parameter in models of ecosystem dynamics and carbon cycle.A method is presented with which instantaneous PAR can be calculated with high accuracy from Moderate Resolution Imaging Spectroradiometer（MODIS） products and Digital Elevation Model（DEM）.The method is based on a simplification of the general radiative transfer equations

which considers five major processes of attenuation of solar radiation:Rayleigh scattering

absorption by ozone and water vapor

aerosol scattering

multiply reflectance between land surface and atmosphere

and three terrain factors:slope and aspect

isotropic sky view factor and additional radiation by neighbor reflectance.Comparing the modeling PAR with observational PAR in Yucheng site and Changbai Mountain site in 2006

the correlation coefficient are 0.929 and 0.904 respectively

and the average errors are 10% and 15% respectively

which should be sufficiently accurate for reflecting the change of PAR.Comparing modeling PAR with the consideration of complex terrain with modeling PAR without the consideration of terrain

this paper quantificationally analyzes the effect of complex terrain.