A novel liquid-level sensor based on a single-mode-multimode-single-mode (SMS) fiber structure is proposed and demonstrated. The transmission spectra of the SMS fiber structure are dependent on the fraction of the length of the multimode fiber that is surrounded by the liquid. Both experimental and numerical results with different liquid levels are presented. The experiment results show that the resonance wavelength exhibits a total blue-shift 4nm with the variation of liquid-level 4mm. Meanwhile transmission optical power is obviously decreases in the wavelength range from 1523nm to 1534nm with the increase of the liquid-level. A measurement sensitivity of 1.02nm/mm and 1.03dB/mm is achieved, respectively.