Grazing and fertilizer addition are critical grassland management practices, influencing plant community structure and even threatening grassland services and sustainability. Besides taxonomic responses, a comprehensive understanding of the impacts on plant phylogeny, functional strategy, and synchrony remains unclear. Taking advantage of Horqin Grassland Ecological Function Zone, Inner Mongolia, China, a random design with 12 8 m x 8 m plots was performed to assess how undergrazing (UG) and fertilizer addition (FA) affect the assembly of grassland in terms of community composition, phylogenetic structure, and CSR strategy (i.e., competitors, stress-tolerators, and ruderals), and the stability of grassland in terms of community diversity and synchrony. Results showed that the UG significantly increased Shannon and Simpson diversity index (33.3 % and 34.7 %, respectively) and significantly altered community composition (F = 4.52) compared with the control (CK). The FA did not exhibit significant taxonomic changes and significantly decreased mean phylogenetic distance (MPD, - 4.40 %) and relatedness index (NRI, -63.13 %), indicating intensified biotic filtering. Fertilized communities exhibited significantly more functional traits associated with competitive strategies (i.e., a C-score increase of 9.85 %, a leaf nitrogen increase of 15.67 %, a leaf carbon decrease of -5.49 %) compared to the CK. Grazed communities exhibited a lower synchrony in species associations (12.9 %) than the CK (19.5 %), while fertilized communities amplified synchrony in species associations (32.3 %) and niche overlap (1.314 vs. 1.026 in CK), thus destabilizing the communities. Our findings highlight that UG sustains diversity and stability, whereas FA prioritizes productivity at the expense of phylogenetic and functional homogenization, exacerbating vulnerability to environmental fluctuations. These results underscore the need for balanced management practices of FA and UG in fragile sandy grasslands to reconcile productivity with long-term ecological persistence.