Abstract: Polycyclic aromatic hydrocarbons (PAHs) existed in equipments, and materials, which would indicate that the lab environment is a potential source of contamination during the pretreatment of samples and consequently affects the accurate analysis of PAHs. A systematic study was undertaken on the sources of naphthalene, acenaphthene, fluorene, phenanthrene and anthracene contaminants during PAHs analysis in water, and a way to remove them successfully. The process of pretreatment was confirmed by the source of some contaminants after excluding uncertainties from data processing and instrument measurement. After examining glass containers, solvents, and the nitrogen condensation system separately, the nitrogen condensation system was found to be the primary source of contaminants. Further investigation confirmed that the nitrogen gas, reductor and pipe used to conduct the nitrogen could contaminate the sample. Finally, these contaminants can be removed by treating the nitrogen gas with an active carbon column after replacing the nitrogen gas, reductor and pipe.