A method to measure salinity concentration in a 10cm-long tapered dual-core As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -Polymethyl methacrylate (As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -PMMA) hybrid fiber with an As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> core diameter of 1.5μm and a PMMA cladding diameter of 88.5μm is proposed. When the salinity concentration is changed in a water bath where the dual-core hybrid taper is placed, the strain on As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> cores by the volume change of PMMA cladding and the refractive index of PMMA cladding are varied, which induce the change of effective refractive index difference between odd and even modes and the wavelength dip shift of the interference spectra in two polarization axes of the dual-core taper. The maximum salinity sensitivities of -0.883nm/(mol/L) and -0.847nm/(mol/L) are experimentally measured in fast and slow axes, respectively, when the salinity concentration changes from 0 to 5mol/L. The salinity sensitivity of the dual-core taper is higher than that of polymer-coated fiber Bragg grating due to the large hygral-induced strain on As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> microwires with small Young's modulus by the volume change of PMMA and the refractive index change of PMMA cladding. The tapered dual-core As2Se3-PMMA hybrid fiber sensor is a good candidate for salinity concentration sensing as high sensitivity, easy to be fabricated and good repeatability.