Based on the Terahertz metal grating resonant transmission phenomenon, a label free biosensor consisting of metallic grating, sample pool and high resistivity silicon substrate was designed by using the sensitive characteristic of surface plasmon resonance of metallic grating for the surrounding medium. Terahertz transmission spectrums of threonine and arginine solutions were measured by using this sensor with terahertz time-domain spectroscopy. The experimental results show that, the resonance frequencies of the threonine and arginine solutions change and shift between 0.6 THz and 0.75 THz with the increase of concentration, moreover, the spectra of the mixed samples of threonine and arginine is not linear superposition of the two spectra.