A highly sensitive detection method based on the evanescent wave of a terahertz subwavelength plastic wire was demonstrated for liquid sensing. Terahertz power spreading outside the wire core makes the waveguide dispersion sensitive to the cladding index variation, resulting in a considerable deviation of waveguide dispersion. Two liquids with transparent appearances, water and alcohol, are easily distinguished based on the waveguide dispersion, which is consistent with theoretical predictions. A melamine alcohol solution with various concentrations is identified successfully, and the detection limit is up to 20ppm, i.e. equivalent to the index variation on the order of 0.01.