Aiming at the problem of the low sensitivity of conventional optical fiber-based gas refractive index sensors, an ultrasensitive tapered optical fiber coupler-based gas refractive index sensor enhanced by the Vernier effect is proposed and demonstrated. The birefringence property of the tapered optical fiber coupler allows it to support two passes of interferences in two orthogonal polarized states, and the superposition of these two interferences forms the Vernier effect. Theoretical analysis and numerical calculations indicate that, for the fiber couplers working in the gas medium when the waist width is within the range of 1.2 to 2.0  μm, the group birefringence difference between the even mode and odd mode equals zero. Thus the sensitivity toward the ambient gas refractive index can be enhanced significantly. To demonstrate these theoretical results, a tapered fiber coupler with a width of 1.6  μm and a length of 16.5 mm was fabricated, and ultrahigh sensitivities up to 22015.4 and −22690.0  nm  /  RIU were experimentally achieved. The proposed sensor has the merits of being easy to fabricate, having compact structure, and being cost effective. It has significant application prospects in the petrochemical and biomedical detection fields.