The foliage clumping index (CI) quantifies the randomness of leaf spatial distribution within a vegetation canopy, which regulates the radiative transfer processes and the energy for photosynthesis. With the recently available global remotely sensed CI products, we deployed CI in the radiative transfer scheme of the Community Land Model version 5 (CLM5) and evaluated the impacts of foliage clumping on global photosynthesis considering the cross-product uncertainties of CI as well as its strong seasonal variations. At the global scale, we find that CI-induced changes of canopy light absorption decreased photosynthesis by 5.9~7.2 PgC/year in sunlit canopy while enhanced photosynthesis by 6.9~8.2 PgC/year in shaded canopy. As a result, shaded canopy increased photosynthesis compensated and exceeded the lost photosynthesis in sunlit canopy, resulting in 1.0±0.12 PgC/year net increase of GPP. Large cross-data CI differences and markedly uncertain CI induced GPP uncertainty were found in tropical regions which was up to 50% of the magnitude of GPP interannual variations. Substantial seasonal differences of the CI impacts on GPP were also found especially over evergreen needleleaf forest (ENF) and deciduous needleleaf forest (DNF). Over ENF and DNF, considering CI but ignoring its seasonal variation showed considerable differences on estimated GPP in the Northern Hemisphere summer. We highlight the need to include foliage clumping, consider its seasonal variation and reduce CI uncertainty and its impacts on modeling global photosynthesis.