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Real-time and on-line monitoring volatile organic compounds(VOCs) are valuable for real-time evaluating air quality and monitoring the key source of pollution. A self-developed proton transfer reaction-mass spectrometer( PTR-MS) was constructed and applied to on-line monitoring trace VOCs in ambient air in Hefei. With the help of a self-developed catalytic converter, the background signal of the instrument was detected and the stability of the instrument was evaluated. The relative standard deviation of signal at m/z 21 was only 0.74% and the detection limit of PTR-MS was 97 part per trillion(97×10–12, volume ratio). As a case of the air monitoring in Hefei, the ambient air at Dongpu reservoir spot was on-line monitored for 13 d with our self-developed PTR-MS. Meanwhile, a solid-phase micro-extraction(SPME) technique coupled to gas chromatography-mass spectrometry/mass spectrometry (GC-MS/MS) was also used for the off-line detection of the air. The results show that our self-developed PTR-MS can be used for the on-line and long-term monitoring of VOCs in air at part per trillion level, and the change trend of VOCs concentration monitored with PTR-MS was consistent with that detected with the conventional SPME-GC-MS. This self-developed PTR-MS can fully satisfy the requirements of air quality monitoring and real-time monitoring of the key pollution sources.