According to the principle of Mach-Zehnder interferometers, an optical in-fiber Mach-Zehnder sensor for Surrounding Refractive Index (SRI) is designed and prepared based on single mode fiber/multimode fiber/single mode fiber/misallied fusion spliced point/single mode fiber structures by misallied fusion spliced technology. The multimode fiber and misallied fusion spliced point in this sensor are served as optical couplers. The light from leading in single mode fiber is coupled into a core mode and cladding modes and propagates in the fiber. Because different modes have different effective refractive indices when the light reaches the misallied fusion spliced point, different modes have different optical path lengths, and the intermodal interference will occur. The dip power of the transmission spectral response to the SRI is theoretically analyzed and experimentally studied. The experimental results show that the dip power at wavelength of 1 530 nm increases monotonically with the SRI in a Refractive Index(RI) range of 1.372 0—1.392 2. Moreover, the response curve of the sensor achieves a good linearity of 0.998 at the RI range of 1.372 0—1.392 2, which is corresponding to a high sensitivity of 252.06 dB/RIU.This compact size, low-cost and highly sensitive SRI sensor is easy to be fabricated, and offers attractive applications in biomedical sensing.