Electrochemiluminescence (ECL) sensing is promising for trace toxic metal ions detection, yet existing signal amplification strategies mostly rely on a single enhancement mechanism or multi-component systems with physically separated enhancers and luminophores, suffering from low energy transfer efficiency and limited sensitivity in complex matrices. Herein, we rationally designed a core-shell nanoluminophore, AgNPs@SiO2@mSiO2-luminol, to achieve synergistic ECL amplification by integrating localized surface plasmon resonance (LSPR) and nanoconfinement effects within a single nanoparticle. The nanoluminophore consists of a AgNP core as the plasmonic source, a dense SiO2 interlayer to suppress metal-induced luminescence quenching while maintaining optimal plasmonic coupling distance, and a mSiO2 shell for high-capacity luminol loading and nanoconfined reaction microenvironment. This architecture yields an 8.4-fold ECL intensity enhancement compared with free luminol, originating from the synergistic effect of LSPR-induced electromagnetic field amplification and nanoconfinement-promoted reactant enrichment and mass transport. By functionalizing the nanoluminophore with thymine, a label-free ECL sensor was fabricated for specific Hg2+ detection via T-Hg2+-T coordination chemistry. The sensor exhibits a wide linear range from 5.86 × 10-7 - 5.86 × 10-2 μg/L and an ultra-low limit of detection of 2.07 × 10-7 μg/L, at least one order of magnitude lower than reported ECL sensors for Hg2+. The sensor shows excellent selectivity, and satisfactory performance in traditional Chinese medicine extracts, consistent with standard inductively coupled plasma mass spectrometry method. This work provides a robust strategy for synergistic ECL amplification in a single nanostructure, and establishes a practical platform for label-free ultrasensitive detection of toxic metal ions in complex matrices.