Chlorine is a highly toxic and hazardous gas, which poses severe risks to environmental safety and occupational health even at low concentrations. Thus, highly sensitive and rapid detection of chlorine at trace levels is essential for air quality monitoring in laboratories, factories, and emergency sites, etc. Herein, a miniaturized optical emission spectrometer was constructed for ultra-sensitive detection of chlorine in air, with absorption-headspace injection. Chlorine gas was first absorbed in a potassium hydroxide solution and subsequently released as molecular Cl2 by a redox reaction with acidic potassium permanganate. The liberated chlorine was accumulated in the sealed headspace vial, and then rapidly transported to the discharge microplasma for subsequent excitation, in which characteristic optical emission of Cl at 837.59 nm was observed and recorded for quantification. Under the selected experimental conditions, a limit of detection of 0.01 ppm (0.03 mg/m3) was achieved, with a relative standard deviation of 3.8% (1 ppm, 2.95 mg/m3, n = 9), meeting the requirements for chlorine gas detection specified in a China’s national standard GB 11984–2024. It was successfully applied for analysis of certified reference materials and real air samples, confirming its accuracy and applicability. Owing to the advantages of high sensitivity, wide linear range, simple operation, and compact design, the proposed method and instrument would be promising for potential on-site detection and monitoring of chlorine in various chlorine operation related scenarios.