Abstract: Air pollution has become more and more serious in China recently, and the volatile organic compounds (VOCs) is a main cause of air pollution. The VOCs also play an important role as the precursors of the ozone and the secondary organic aerosol (SOA). A high-resolution photoionization time-of-flight mass spectrometer (TOF MS) based on the vacuum ultraviolet (VUV) lamps was developed, and a new method for dilution sampling of VOCs was established. The developed dilution sample method directly fed the sewage discharged from the high temperature and high pressure vessel into the sampling bottle after buffering. Firstly, an analysis and research was carried out for the process of the wastewater of a refinery in transport, storage and emissions, and then this method was used in the sampling and analysis of VOCs in the petrochemical industry sewage treatment system. According to the layout of the sewage treatment system in the petrochemical enterprise, general environmental protection sampling points were selected. In addition, sampling points were set up upstream and downstream of the plant area, and then sampling points were determined according to field investigation. The results showed that no high-concentration VOCs were detected in the three tanks of oil separation system. Among them, dissolved air flotation toluene had the highest mass concentration (50.41 mg/L), while the mass concentration of other components ranged mostly from 0.1 to 1.0 mg/L. Similar component concentrations were found in 1# hydrogenation and 2# hydrogenation oil separation tanks. Take 1# hydrogenation oil separation tank as an example, the mass concentration of toluene is 10.60 mg/L, ethylbenzene is 52.33 mg/L, and benzene is 59.80 mg/L and so on. Because the oil separating tank in tank area was not capped and in open air, the VOCs was easy to diffuse into the atmosphere, and no higher concentration components were detected. The results also showed that new sampling method effectively diluted the quantitative sample, the operation was simple, and the loss of the VOCs due to the flashing process was reduced, so the detected substances were more abundant. At the same time, the self-developed TOF MS can be used to detect the sewage treatment system of different devices quickly. The results showed that most of the closed cells of chemical enterprises contained benzene series and ethers, but there were significant differences in the VOCs from different processes. Mass spectrometer can be well applied to the monitoring of VOCs in the petrochemical enterprises’ sewage systems.