In this study, a multi-wavelength spectrophotometric method for simultaneous determination of peracetic acid (PAA) and coexistent hydrogen peroxide (H₂O₂) was presented. This method was based on the oxidation of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) with the assistance of Fe²⁺/KI to produce a stable green radical (ABTS^●+), which could be determined at four characteristic peaks (i.e., 415 nm, 650 nm, 732 nm, and 820 nm). The absorbances of ABTS^●+ at four peaks were well linear (R² > 0.999) with concentrations of both total peroxides (PAA + H₂O₂) and PAA in the range of 0–40 μM under optimized conditions. The sensitivities for determining total peroxides at 415 nm, 650 nm, 732 nm and 820 nm were determined to be 4.248 × 10⁴ M⁻¹ cm⁻¹, 1.682 × 10⁴ M⁻¹ cm⁻¹, 2.132 × 10⁴ M⁻¹ cm⁻¹, and 1.928 × 10⁴ M⁻¹ cm⁻¹, respectively. For determining PAA, the corresponding sensitivities were 4.622 × 10⁴ M⁻¹ cm⁻¹, 1.895 × 10⁴ M⁻¹ cm⁻¹, 2.394 × 10⁴ M⁻¹ cm⁻¹ and 2.153 × 10⁴ M⁻¹ cm⁻¹, respectively. The concentration of coexistent H₂O₂ was gained by deducting PAA concentration from total peroxides concentration. The ABTS method was accurate enough to determine PAA concentration in natural water samples. Moreover, the ABTS method was successfully used to determine the changes of PAA and coexistent H₂O₂ and to distinguish their role on naproxen degradation in heat-activated PAA process. Overall, the ABTS method could be used as an alternative method for the convenient, rapid and sensitive determination of PAA and the coexistent H₂O₂ in water samples.