Abstract: A series of spectral measurements were made at eleven sites in eight regions to determine the photosynthetically active direct radiation between 0.4 and 0.7μ. From the measurements of 2878 sets an empirical formula for estimating the relative flux density ηS of the photosynthetically active direct radiation has been obtained. The climatological methods for estimating the relative flux densities, ηD and ηO, of the photosynthetically active diffuse radiation and the global radiation under various sky conditions were suggested.In terms of the solar radiation data of eight representative stations in China the relative flux densities in January, April, July and October were calculated, The monthly mean values calculated at eight stations are as follows ηS=0.39±0.04, ηD=0.49±0.04 and ηO=0.44±0.03.