The construction and the performance of the newly developed proton transfer reaction time of flight mass spectrometer (PTR-TOFMS) are reported. Firstly, the threshold voltage of the discriminator on data acquisition board was optimized. The results show that when the threshold voltage is 0.33 V, the intensity ratio of H316O+ (m/z 19) and H318O+ (m/z 21) is about 500:1. Then the relationship between the distribution of water clusters ion H3O+ (H2O)n=0,1,2,3 and the reduced electric field in drift tube was studied. The results show that when reduced-field (E/N) is 139 Td (1 Td=10－17 Vcm2), the water cluster ions are well suppressed, and the purity of the ion H3O+ is more than 99%. The resolutions at different m/z were also investigated. The results show that the best resolution is at m/z=124 with RMAX=2653. The performance of the PTR-TOFMS instrument was tested by using different concentrations of ethanol gas. The results show that the linear dynamic range of the PTR-TOFMS instrument can reach three orders of magnitude, and the detection limit can reach 3.8 ppb. At last, the PTR-TOFMS was used to detect the laboratory air to get a mass spectrum of different volatile organic compounds (VOCs). It is indicated that the newly developed PTR-TOFMS can be used for on-line monitoring trace volatile organic compounds, which will be of great value in the fields of environment, food, medicine and other fields.