In order to deeply understand whether P addition could alleviate the N deposition-induced decrease in plant community diversity, we explored the effects of P addition on plant biomass, community diversity, and soil C:N:P ecological stoichiometry under 2-year of 5 g/m²/a of N addition (2015-2016) in a long-term fenced desert steppe, located in Yanchi County, Ningxia, north-western China. We also analysed the relationships between plant community diversity indices and soil C:N:P ecological stoichiometry as well as other key soil factors. The results showed that combined with low N addition, low P addition promoted plant biomass accumulation, but moderate and high P addition inhibited the growth of most species, resulting in decreases in the importance values of Lespedeza potaninii, Astragalus melilotoides, and Sophora alopecuroides. With the increase of P addition, both Shannon-Wiener diversity index and Patrick richness index increased firstly and then decreased, while there were gradual increase in Simpson dominance index and little change in Pielou evenness index. Increasing P addition reduced soil C:P and N:P ratios; soil N:P ratio, C:P ratio, total P content, available P concentration, and microbial biomass C:P ratio were tightly related to plant diversity indices, indicating that the decoupling of elemental stoichiometry in soils will exert great impacts on plant community composition under increasing N deposition. Taken together, these results might suggest an appropriate amount of P addition could regulate the pressure between soil P supply and plant P demand through increasing soil availability, raising litter return amount, stimulating microbial P release, and other approaches, to further alleviate the N addition-induced decrease in plant community diversity.