All of the commercialized electrochemiluminescence (ECL) immunoassays are automatically conducted at +1.40 V (vs Ag/AgCl) in the coreactant route. To alleviate the exogenous effect of coreactants and simplify the operation procedures, herein, a sulfur-vacancy-involved and free electron strategy is proposed to exploit Au nanoclusters (NCs) as anodic electrochemiluminophores and perform a coreactant-free immunoassay. The deficient coordination between the sulfhydryl of Met and the Au core might induce the departure of partial S atoms and enable Met-capped AuNCs (Met-AuNCs) with a sulfur-vacancy-involved electron-rich nature. The electron-rich nature tends to endow Met-AuNCs with unpaired endogenous free electrons, which can directly combine exogenous holes for light emitting. Coreactant-free ECL at around +0.86 V is consequently and conveniently achieved by merely oxidizing Met-AuNCs at the anode. The coreactant-free ECL is qualified to determine human carcinoembryonic antigen from 10 to 5000 pg/mL with a limit of detection of 5 pg/mL. Electron paramagnetic resonance provides clear evidence that endogenous free electrons within Met-AuNCs play an important role in the generation of coreactant-free ECL. This sulfur-vacancy-involved and free electron strategy is promising for designing nanoelectrochemiluminophores with improved immunoassay performance.