In this work, terahertz time-domain spectroscopy(THz-TDS) was used to measure the content of hydrocarbons in alkylate gasoline as a new kind of non-destructive testing. 2,4,4-trimethyl-pentene(2,4,4-TMP), a representative kind of olefin, was mixed in alkylate gasoline. The spectral response of the samples containing different 2,4,4-TMP in terahertz band was analyzed. The results showed that amplitude of samples had a significant change in signals and varied linearly with content of 2,4,4-TMP changed. The curves, fitting by maximum and minimum values(amplitude) of spectra, were used to predict two unknown sample concentration. As a result, the error is less than 0.5%. Meanwhile, the absorption coefficient of samples were analyzed. The research indicates that THz -TDS technique is very suitable for detection of composition and content of hydrocarbons in gasoline.