Terahertz time-domain spectroscopy (THz-TDS), a newly developed far-infrared technique in recent years, has been widely applied for the detection of polar gas and non-polar gas. The principal components in distillation, natural and marsh gas include methane (CH4), ethane (C2H6) and propane (C3H8). The three kinds of alkane gas are separately measured for the frequency domain and phase spectra, and then mixed as a series of two-component systems which are finally measured with THz-TDS. The frequency dependent amplitude and phase spectra prove that C3H8 has stronger absorption in THz range compared with CH4 and C2H6. Such trend is agreement with the polarity intensities of alkanes. In addition, back propagation artificial neural network (BPANN) is employed to quantitatively analyze the contents and total pressure with the frequency-amplitude spectra as the input. The correlation coefficients R between actual and predicted values are calculated in both training and prediction sets, where r range from 0994 to 0999 and from 0981 to 0993, respectively. Therefore, the combination of THz-TDS and BPANN can realize the quantification of multiple properties in alkane mixtures, which would promote the application of THz-TDS technique for gas detection.