A fiber-optic Mach Zehnder interferometer (MZI) refractive index (RI) sensor based on a double peanut-shaped structure in Er-doped fiber with a section of tapered fiber was fabricated using fusion splicer and experimentally demonstrated. The double peanut-shaped structures placed at both ends of the sensor function as beam splitting/combining. Besides, the tapered region works as the sensing area. With RI changes, the optical path difference between the core mode and the cladding modes will change correspondingly, resulting in interference spectrum shifting. By monitoring the shift, the measured RI can be obtained. The experimental results show that the RI sensitivity was significantly improved compared with other types of peanut-shaped fiber sensors. By comparing different taper diameters, it is found that the finer the tapered fiber, the higher the sensitivity of measuring RI.High RI sensitivity of 441.56 nm/RIU is obtained in the experiments. Compared with the similar-structure sensor consisted of single-mode fiber (SMF), the sensitivity of the proposed sensor based on Er-doped fiber is 115.22 nm/RIU higher than that of the former. The proposed sensor has the characteristics of a compact structure. In addition, it has potential in environmental, medical, and aerospace monitoring applications.