To gain further insight into how changes in grassland species diversity affect aboveground productivity in response to nutrient addition, this study conduced nutrient addition experiments in an alpine meadow on the Qinghai-Tibet Plateau. By investigating plant communities and measuring functional traits, this study aimed to analyze the mechanism by which nutrient addition drives aboveground productivity, based on functional traits and species diversity. The results indicated that: (1) Nitrogen addition alone and combined nitrogen-phosphorus addition significantly increased the community's aboveground productivity by 67% and 49.1%, respectively, suggesting nitrogen is the primary limiting nutrient for the aboveground productivity. (2) Nitrogen addition alone and combined nitrogen with phosphorus resulted in a notable reduction in species richness, the Shannon-Wiener index, and functional richness. (3) Plant height significantly increased by 24.2% following nitrogen addition alone and by 29.6% with combined nitrogen-phosphorus addition. Additionally, the specific leaf area significantly increased by 22.1% with phosphorus addition alone and by 34.6% with combined nitrogen-phosphorus addition. The leaf dry matter content significantly increased by 10.1% in response to nitrogen addition, while the leaf chlorophyll content notably decreased by 5.5% under phosphorus addition. (4) No significant linear regression relationship was found between aboveground productivity and functional diversity. However, a significant negative linear regression relationship was found with species diversity, while a positive relationship was observed with leaf dry matter content. (5) Functional diversity and functional traits have limited explanatory power for aboveground productivity, whereas species diversity provides a more effective explanation for the changes in aboveground productivity. This finding is consistent with the predictions of the mass ratio hypothesis. In conclusion, this study shows that the increase in aboveground productivity of alpine meadow communities under medium-term nutrient addition conditions is primarily driven by the functional traits of dominant species, and species diversity provides a better explanation for the changes in aboveground productivity of alpine meadow in comparison to functional diversity.