Restoration of degraded grasslands can enhance productivity and promote carbon and water cycling, but changes in energy fluxes and the partitioning of net radiation (Rn) during restoration remain poorly understood. Clarifying the role of vegetation structural and physiological regulations of energy partitioning in semiarid grasslands is critical for advancing our understanding of ecosystem functioning during grassland restoration and its feedback to climate. In this study, we investigated the seasonal and interannual variations in energy fluxes and Rn partitioning over thirteen years (2006–2018) in a long-term fenced semiarid grassland site using eddy covariance method. Our results revealed a rapid shift from sensible heat flux (H)-dominance to latent heat flux (LE)-dominance during the early growing season, aligning with the phenological development of vegetation. This transition produced d a U-shaped seasonal pattern in the Bowen ratio (β, i.e. H/LE). During the H-dominated period, Rn alone governed LE and H. In contrast, during the LE-dominated period, vegetation exerted strong control over energy partitioning through both structural (leaf area index, LAI) and physiological (canopy conductance, gc) pathways, mediated by climatic factors. The seasonal distribution of precipitation and snowmelt-derived soil moisture, combined with gc, primarily regulated interannual variability in LE, H, and β. Notably, vegetation physiology, mediated by gc, exerted significant influence control over energy exchange at both seasonal and interannual scales. Furthermore, vegetation restoration enhanced available energy but did not significantly alter LE and H fluxes. This study highlights the substantial impact of vegetation growth on energy balance in grassland ecosystems under long-term exclosure, emphasizing the need to integrate these dynamics into future sustainable management practices.