This study aimed to develop a novel method for the green synthesis of selenium nanoparticles (SeNPs) using an aqueous extract of Kyllinga brevifolia, providing a safe and efficient nanomaterial preparation technology for food preservation. The extract was prepared by the water extraction method and reacted with sodium selenite solution at 100℃. After 48 h of incubation, the product was purified by centrifugation and freeze-dried to obtain SeNPs. The nanoparticles were characterized using gas chromatography-mass spectrometry (GC-MS), ultraviolet-visible (UV-Vis) spectroscopy, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and dynamic light scattering (DLS). Their antioxidant activity was evaluated by DPPH and ABTS radical scavenging assays. The results showed that the reaction solution changed from transparent brownish-red to turbid brick-red, with a maximum absorption peak at 407 nm. SEM revealed uniform spherical nanoparticles. XRD patterns exhibited diffraction peaks at 2θ angles of 21.80°, 23.74°, and 29.92°, consistent with the JCPDS standard. DLS measurements indicated an average particle size of 66.44 nm, a polydispersity index (PDI) of 0.1153, and a zeta potential of -16.69 mV. GC-MS identified 17 active components, among which linoleic acid, oleic acid, and 4-hydroxy-2,5-dimethylfuran-3(2H)-one-containing C=C bonds and -OH groups-reduced Se4+ to Se0 and exerted capping and stabilizing effects. The DPPH and ABTS radical scavenging assays demonstrated concentration-dependent antioxidant activity of the SeNPs. This study successfully transformed an invasive weed into functional nanomaterials. The established green synthesis method is simple, environmentally friendly, and low-cost, laying a theoretical foundation for the application of SeNPs in fruit and vegetable preservation and functional food development.