Many studies have explored changes in ecosystem functions related to global environmental changes, but little is known about the mechanisms and pathways behind the changes. Productivity is an important component of ecosystem functions. However, it is not clear how change and the mechanism of change of the primary productivity of desert grassland plant communities under nitrogen (N) addition; does N affect the productivity of desert grassland by affecting biodiversity? A manipulative N addition experiment located in the desert grassland has been conducted since 2018. The experimental treatment included five levels of N addition rate (i.e., control, 5, 10, 20 and 40 g m^-2 a^-1) with four replicates for each treatment. The plant functional traits, species richness and composition, and aboveground productivity were determined. In addition, species diversity, functional diversity and community weighted average traits were calculated. The results showed that: (1) the functional diversity (Rao index), community weighted average-height (CWM-height), functional evenness and functional dispersion of the desert grassland increased significantly in the fourth year of N addition treatment (2021), while the species richness and Shannon-Wiener index of the desert grassland decreased significantly. (2) N addition could indirectly promote the primary productivity of desert grassland by affecting species richness and functional diversity, but the impact of CWM-height on the primary productivity was a positive effect, while species richness and functional dispersion had a negative effect on productivity. (3) The biomass ratio hypothesis could better explain the variation of the primary productivity of desert grassland communities. Our results indicated that N addition regulated the primary productivity of desert grassland mainly by altering the functional traits of dominant species within the community.