Chlorine radicals (Cl·) can strongly affect the fate of chemical composition in the atmosphere. In the polluted troposphere, it can increase concentrations of ROx (OH, HO₂ and RO₂) radicals by oxidizing volatile organic compounds (VOCs), thereby perturbing the formation of O₃. However, related studies on reactive Cl species in the marine areas of China remained limited. Here, molecular chlorine (Cl₂) and nitryl chloride (ClNO₂) were simultaneously detected using chemical ionization mass spectrometry (CIMS) in Xiamen, a coastal city of southeast China during the autumn in 2022. High concentration levels of Cl₂ and ClNO₂ were observed and the diurnal Cl₂ concentrations presented a peak at noon, indicated that in urban Xiamen, Cl₂ has significant source in the daytime. Meanwhile, ClNO₂ showed a strong diurnal variation with higher levels in the morning and evening and lower levels in the afternoon. Moreover, based on an observation-based model combining with the Master Chemical Mechanism, we evaluated Cl· budget and the influence of Cl· on atmospheric oxidation capacity, ROx radicals and O₃ production. Our study revealed the significant role of reactive Cl species in the marine atmosphere of southeast China, and took control of chlorine precursors into account in O₃ pollution control.