Chlorobenzene compounds and their high-temperature pyrolysis products are one kind of the important pollutants in the atmosphere. The research on the pyrolysis path of chlorobenzene can provide basic theoretical guidance for preventing or reducing the emission of related pollutants. The pyrolysis of chlorobenzene in a flow tube reactor was studied at 4 kPa and 873～1373 K by in-situ synchrotron radiation photoionization mass spectrometry (SR-PIMS). 16 products including olefin, aromatic hydrocarbons, and chlorinated aromatic hydrocarbons were detected in real time. Mole fractions of these products were obtained as function of pyrolysis temperature. The results show that o-benzyne is a key intermediate for the formation of small molecules in the pyrolysis of chlorobenzene. Besides, compared with the pyrolysis results of chlorobenzene at atmospheric-pressure observed before in this group, it was found that low pressure can reduce the formationpathways of chlorine-containing organics such as chlorinated aromatic hydrocarbons, an important secondary pyrolysis product of chlorobenzene, which indicates that pressure conditions can affect the formation and the following atmospheric emission of chlorinated aromatic hydrocarbons.