An excessively tilted fiber grating (ExTFG) Sagnac interference loop refractive index sensor based on vernier effect (VE) is proposed. Utilizing the basic principle of the optical VE, a polarization-maintaining fiber Sagnac interferometer with a free spectral range (FSR) of 1.52 nm and a Sagnac interferometer composed of ExTFG with an FSR of 1.36 nm are theoretically designed and fabricated. The vernier spectrum is obtained by superimposing two optical paths by cascading. Experimental results show that the characteristics of the vernier envelope are easy to identify, and the refractive index sensitivity reaches -1286.40 nm/RIU through vernier amplification, improved ~8.46 and ~10.55 times than that of the TM and TE polarizations of the ExTFG sensors, respectively, which is in good agreement with the theoretically calculated result. The output spectrum of the sensor is relatively stable, and the sensor has the advantages of simple structure, convenient fabrication, and strong resistance of temperature crosstalk, leading to promising application in the fields of biochemical and biomedical detections.