A flow-injection chemiluminescence (CL) system with high sensitivity, selectivity, rapidity, and reproducibility is proposed for the determination of hydrogen peroxide (H₂O₂) in water samples. The system is based on the reaction of hydrogen peroxide and hydrogen carbonate solution. Carbon nanospheres (CNSs) prepared from aqueous glucose solution are used to enhance the weak CL. The CL intensity was found to be directly proportional to the concentration of H₂O₂ present in the sample solutions. The effects upon the CL of several physicochemical parameters, including the concentration of the reagents, the mixing order of the reagents, flow rate, pH, particle size of CNSs and other relevant variables, were studied and optimized. The proposed method exhibited advantages in a larger linear range of 5.0 × 10⁻⁸ to 3.0 × 10⁻⁶ mol L⁻¹ and a lower limit of detection of 1.0 × 10⁻⁹ mol L⁻¹ (S/N = 3). This method has been successfully applied to the evaluation of H₂O₂ in tap water and snow water with recoveries from 80 to 110%. The relative standard deviation (RSD) was less than 8% for intra- and inter-assay precision. Based on the kinetic curve, the CL spectrum, fluorescence spectrum, UV-visible spectrum, and electron spin resonance (ESR) spectrum of NaHCO₃–H₂O₂–CNSs system, a possible CL mechanism was proposed. Superoxide ion radical (˙O₂⁻) and hydroxide radical (˙OH) were generated during the reaction of NaHCO₃ and H₂O₂. They were the key intermediates for the production of hole-injected and electron-injected CNSs in the CL process.