Abstract: The strong localized plasmon resonance of metasurfaces makes the resonance frequency extremely sensitive to the dielectric environment, which can be applied to label-free environment detection. In this paper, a bow-tie terahertz metasurface with an optimized ratio of the quality factor to effective mode volume(Q/V_eff) is designed. The unit cell of the proposed structure is composed of a mirror-symmetrical metallic bow tie in the middle and continuous metallic strips on both sides. The width of each metal strip and the length of the bow-tie gap are optimized for the parameter Q/V_eff. When the metal strip width is 25 μm and the gap length is 2 μm, the effective mode volume is 3.6 μm³ and Q/V_eff is 2.2 μm^-3 at 0.7 THz. In the experiment, different concentration of the lead ion solution was dropped on the proposed metasurface. The transmission spectrum was measured by a terahertz time-domain spectroscopy system. The results showed that there is a linear relationship between resonance frequency shift and lead ion solution concentration from 0.1 ng/mL to 20 ng/mL. The detection limit is 0.1 ng/mL. The terahertz metasurface sensor has the advantages of the miniaturized size, easy sample preparation, fast measurement capability and real-time detection, which will be widely used in environmental protection and food safety.