l-Cysteine (l-Cys) was added to EGSB (expanded granular sludge bed) reactors during the startup process. A parallel EGSB without l-Cys addition was started as a control. The results showed that the startup process of the EGSB with l-Cys supplementation was well accelerated compared to the control reactor (without supplementation). Supplementation of l-Cys every four HRT (hydraulic retention time) had shortened the startup time. Hydrogen production rate was stimulated by 23.7% when adding 0.5 g/L l-Cys to the EGSB. The maximum hydrogen production yield was 1.93 mol/mol glucose, which was 20.6% higher than the control. The main liquid metabolites were ethanol and acetate. The substrate utilization rate was also increased. Scanning electron microscopy observations showed that obvious micro channels formed in the l-Cys supplemented reactors, which might well promote the mass transfer and energy exchange. The results showed that l-Cys supplementation was a feasible method to accelerate the continuous reactor startup.