A mid-infrared gas sensor based on a tapered sub-wavelength grating slot waveguide comprises a lower cladding, a first tapered grating array and a second tapered grating array. The first tapered grating array and the second tapered grating array are disposed on an upper surface of the lower cladding. The first tapered grating array is located in front of the second tapered grating array. The first tapered grating array is formed by 5566 identical first core waveguides that are regularly distributed at intervals from left to right. The second tapered grating array is formed by 5566 identical second core waveguides that are regularly distributed at intervals from left to right. The first core waveguides and the second core waveguides are tapered waveguides. Upper sides and lower sides of the first core waveguides and the second core waveguides are isosceles trapezoids.