A fiber-optic solution concentration sensor based on a pressure-induced long-period grating (LPG) in a composite optical waveguide is proposed.The composite waveguide consists of a standard single-mode fiber with its coating stripped away,a teflon-cannula and the medium to be measured.An experiment has been carried out to measure the concentration of the sodium chloride (NaCl) solution.The results show that the central resonant wavelengths of the LPG shift towards shorter wavelengths when the concentration of the NaCl solution increases.The central resonant wavelength of the LP14 cladding mode exhibits a total blue shift of 4.13nm in the NaCl solution concentration range of 0-25％,which corresponds to a sensing sensitivity of 0.17nm/％.