Display omitted •The high-sensitivity terahertz spectrometer for detecting trace gas and quantitation.•Superior spectral specificity was demonstrated by comparing with the JPL database.•Limit of detection of terahertz heterodyne spectrometer for CH3CN is obtained.•Quantitation method based on collision broadening for trace gas is proposed. Quantitative analysis of trace gases is an important research field in analytical chemistry. The terahertz electronic spectrometer is one of the most powerful tools for detecting trace gas. Here, a terahertz spectrometer based on frequency multiplier chain and heterodyne detection was presented. The rotational spectra of acetonitrile (CH3CN) gas were measured in the 290–370 GHz frequency band with 100 kHz spectral resolution. The spectrometer demonstrated excellent spectral specificity and the extrapolated limit of detection for CH3CN gas of 1.4 ppm. Furthermore, a novel quantification method of trace gas was proposed based on broadening mechanisms. The CH3CN self- and nitrogen (N2)- collisional broadening coefficients were obtained experimentally for verifying the method. The CH3CN concentration of the validation group was calculated, and the relative error was 0.1%. The error analysis of the different number of measurements of the method was carried out. The method could provide a new perspective for trace gas quantitative analysis.