Long-term soil erosion causes widespread nitrogen (N) and phosphorus (P) limitation on the Loess Plateau. The response of community biomass to N and P addition is tightly linked to interspecific and intraspecific variations in community level traits. To explore this relationship, we conducted a split-plot experiment with N and P addition in the semiarid grasslands of the Loess Plateau. Measured key functional traits included plant height (PH), specific leaf area (SLA), and leaf/root N and P contents (LN, LP, RN, RP) with their ratios. Results showed that both single N and combined N and P addition significantly increased the community-weighted mean (CWM) of multiple key traits. Further analysis revealed that under N and P addition, variations in CWM traits were mainly driven by intraspecific variation. Intraspecific variation in CWM of PH, intraspecific variation in CWM of LN, and interspecific variation in CWM of LP were the key driving factors for changes in biomass. Our results highlighted that intraspecific variation primarily promoted community biomass accumulation by driving CWM traits under N and P addition. The key role of intraspecific variation contributes to an accurate understanding of the potential driving mechanisms underlying community biomass changes under nutrient enrichment, thereby providing a theoretical basis for the sustainable improvement of productivity during the restoration of grasslands on the Loess Plateau.