Understanding the effects of reseeding on degraded grassland community characteristics and population niche dynamics holds critical importance for grassland ecological restoration. In degraded desert grasslands of Ningxia, using enclosure grassland as the control (CK), we selected Agropyron desertorum, Agropyron mongolicum, Lespedeza pataninii, Astagalus laxmannii, and Astragalus melilotoides as reseeding species. Six combinations of native grass species were established for reseeding at a total Poaceae-to-Fabaceae seed weight ratio of 2 ∶ 1. We investigated the plant community characteristics and population niches in the desert grassland several years after reseeding. Through comprehensive analysis of plant community characteristics and niche dynamics six years post-reseeding, we found: 1) All reseeded combinations exhibited enhanced quantitative vegetation characteristics compared to CK, although CK maintained superior species diversity indices (Margalef richness, Shannon-Wiener diversity, and Simpson dominance indices) except for Pielou evenness. 2) Niche width values exceeding 2 were observed in >50% of species across all combinations except A. mongolicum+A. laxmannii. Despite prevalent interspecific resource competition, treatments showed reduced proportions of species pairs with significant niche overlap (Q_ik ≥ 0.5) relative to naturally enclosed grassland. 3) Interspecific association patterns varied substantially among combinations. The A. desertorum+A. mongolicum+A. melilotoides combination demonstrated predominantly positive associations, whereas A. desertorum+A. mongolicum+L. potaninii showed primarily negative associations. All combinations contained more negatively associated species pairs than positive ones. 4) Structural equation modeling revealed that soil available phosphorus, total nitrogen, and pH drove community structure and niche differentiation through both direct and indirect pathways. Spatial heterogeneity mediated diversity patterns via resource partitioning and competitive interactions. Based on integrated assessment of community characteristics, niche dynamics, and species interactions, we recommend A. desertorum+A. mongolicum+A. melilotoides as the optimal reseeding combination for restoring degraded desert steppe ecosystems in the study region.