In this paper, we carried out theoretical and experimental research on the structure of a side-polished fiber Mach-Zehnder interferometer (MZI) based on a microtaper. To be specific, after the side-polished depth of the fiber was appropriately controlled, the refractive index (RI) sensing performance of the MZI was much better than that of a traditional fiber MZI without side polishing. Besides, the research results showed that when the side-polished depth reached 41.7 μm, the sensitivity of RI sensing was -117.145 nm/RIU at the RI of about 1.34. Furthermore, we measured the temperature and humidity simultaneously by depositing the hydrophilic graphene oxide (GO) on the surface of the side-polished fiber MZI, obtaining the sensitivity of temperature and relative humidity sensing of 131.77 pm/℃ and -76.1 pm/%RH, respectively. In conclusion, the proposed side-polished fiber MZI has the advantages of high sensitivity, low cost, and simple fabrication, thus displaying a wide range of application prospects in the field of biochemical sensing.