Abstract: Chlorobenzenes (CBs), one of the typical precursors of dioxin in the exhaust gas during waste incineration, can be considered as the indicators for on-line monitoring of dioxin. Based on windowless RF discharge, a single photon ionization time-of-flight mass spectrometer with high photon density was designed to detect the chlorobenzenes with low concentrations in exhaust gas during waste incineration. With the combination of automatic trap and release system, the on-line and rapid detection of chlorobenzenes was investigated. With krypton as the discharge gas, windowless RF discharge photoionization source was employed instead of commercial vacuum ultraviolet (VUV) lamp, which not only improved the radiation efficiency of photons as well as the sensitivity of instrument, but also avoided the pollution of lamp window. In the single measurement period of 36 min, a TA trap tube that filled with 200 mg Tenax was used to implement the sample preconcentration for 10 min, resulting in the linear response ranges of 0.5-50, 0.64-65 and 0.8-80 ng/L for monochlorobenzene, 1, 3-dichlorobenzene and 1, 2, 4-trichlorobenzene, with limit of detections (S/N>10) of 6.4, 9.7 and 15.2 pg/L, respectively. From the continuous monitoring result for 6 h, the relative standard deviations (RSD) of signal intensities for monochlorobenzene, 1, 3-dichlorobenzene and 1, 2, 4-trichlorobenzene are obtained to 7.5%, 5.8% and 11.1%, respectively, demonstrating the good stability of windowless RF discharge single photon ionization time-of-flight mass spectrometer, which can be applied in the on-line monitoring of typical precursors of dioxin in the exhaust gas during waste incineration dioxin.