Abstract: To study the impact mechanism of chloride ion (Cl⁻) on the remediation of phenol-contaminated groundwater by persulfate (PS) advanced oxidation technology. The effects of temperature, PS concentration, initial pH value, and Cl⁻ concentration on the degradation of phenol were investigated by thermal activated PS advanced oxidation technology. The spectral characteristics of the reaction solution during phenol degradation were identified by parallel factor analysis. Moreover, the quantities and species of chlorinated toxic by-products were identified by gas chromatography coupled to mass spectrometry, and the degradation mechanism was revealed. The results showed that: (1) Degradation of phenol was promoted by increasing the reaction temperature and PS concentration, and the degradation process was in accordance with the pseudo-first-order kinetics model. (2) The presence of Cl⁻ accelerated the degradation of phenol by thermal activated PS, and the degradation efficiency increased with the increase of Cl⁻ concentration. When Cl⁻ concentration was retained at 10, 25, and 50 mmol/L, the degradation rate of phenol was reached 100% after the reaction of 5 h. (3) The fluorescence characteristics of the reaction solution can be divided into a four-component model (C1, C2, C3 and C4) during the degradation of phenol. Fluorescence intensity of C1 and C2 components decreased more significantly in the presence of Cl⁻. C3 and C4 components are assigned to the degradation products of phenol, and the fluorescence intensity of C3 increased first and then decreased during the reaction. (4) Seven chlorinated toxic by-products (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, chlorohydroquinone, 3,5-dichlorocatechol, 2,3-dichloro-2-methylbutane, and 2-chloro-4-methyl-2-pentanol) were detected, and the degradation mechanism was proposed using the mass spectrometry, including hydroxylation/oxidation and chlorination process. The study showed that Cl⁻ can promote the remediation efficiency of thermal activated PS for phenol-contaminated groundwater, but also induce the generation of chlorinated toxic by-products due to the chlorination process, which will provide support for environmental safety evaluation of PS advanced oxidation technology for phenol-contaminated groundwater.