Abstract: A method has been developed for the simultaneous detection of 27 volatile organic compounds (VOCs) including halogenated hydrocarbons, benzene series compounds and chlorinated benzenes in groundwater by Gas Chromatography-Mass Spectrometry (GC-MS) with purge and trap sampling. The double capillary columns of DB-624 and DB-FFAP were used for identifying the target compounds based on the complexity of the environmental medium and uncertainty of interference. Parameters affecting the accuracy and precision, such as separation conditions, injection mode and mass spectrometry scanning mode, were tested and optimized. The peak shape of target compounds in two different injection modes was compared and the results show that the split injection mode is significantly better than the non-split mode. The split injection with a ratio of 10:1 was selected to determine the target analytes with high sensitivity. The selected ion monitoring (SIM) scan mode improved the sensitivity of the components within the scan cycle. The double capillary columns of DB-624 and DB-FFAP were used to identify the target compounds, which ensured the accuracy for the analysis results. Meanwhile the m-xylene and p-xylene were totally separated by DB-FFAP capillary columns. The detection limits of the method for the target compounds were 0.02-0.10 μg/L. The recovery rates were 93.3%-109.5% with RSD of 0.9%-8.5% (n=7). The method has the advantages of lower detection limits, good precision and is suitable for the determination of trace VOCs in groundwater for a large number of samples.