A novel fluid sensing system based on side-polished optical fiber (SPOF) is proposed, which realizes the fluid replaceability and effective refractive index (RI) sensing characteristics. Numerical investigations demonstrate that the photonic bandgap effect can be obtained if the RI of liquid is higher than that of substrate material in the wavelength range studied. The relationship between bandgap edge wavelength and RI is studied theoretically. The SPOF with a depth of 57 μm is used in the experiment to realize the construction of the fluid channel. After filling three different liquids, the result shows that the wavelength of the bandgap edge has a red shift with RI increased, which is nearly linear in the RI range of 1.56—1.6 with a sensitivity about 5 543.64 nm/RIU. The proposed sensing system can be flexibly applied to the field of fluid characteristic sensing such as biochemical solution characteristic detections.