A new method for measuring terahertz (THz) transmission spectra of bio-chemical thin films placed on the surface of a semiconductor by terahertz surface plasmon polaritons (SPPs) was proposed. A strongly confined surface plasmon in the terahertz frequency range on the surface of semiconductor was theoretically proved, which can be used to enhance the light-matter interaction with bio-chemical layers above the semiconductor surface. By employing a free-space THz time-domain spectroscopy (TDS) system, the transmission power spectra of onion epidermis films with free space transmission and SPP transmission, respectively, were experimentally obtained. Experimental results show that SPP transmission spectra represent many characteristic absorption peaks for a layer of onion epidermis film, which are more than 100 times thinner than the free space wavelength of the terahertz wave.