An optical fiber sensor based on metal-organic frameworks (MOFs) was designed and fabricated. It consisted of a section of no-core fibers (NCFs) spliced between two single-mode fibers (SMFs) and a ZIF-8 nanofilm coated on the surface of the NCFs with the in-situ crystallization. When an optical signal was coupled from SMFs into NCFs, a variety of high-order modes were excited and multimode interference took place. The interference result was exceptionally sensitive to changes in the refractive index of the environment surrounding the NCFs. The porosity of the ZIF-8 material and its specific adsorption ability for ethanol molecules were availed. The adsorbed ethanol molecules filled the pores of the ZIF-8 nanofilm, which resulted in changes in the refractive index of the nanofilm and the condition for multimode interference in the NCFs. Consequently, shifts in the transmission spectrum occurred. Trace ethanol in an aqueous solution was thereby detected. The experimental results show that the sensor has a sensitivity of 1.3 nm·% ^-1 and a lower detection limit of 1% (ethanol/aqueous solution, volume fraction). The proposed optical fiber sensor, with a simple structure and being easy to prepare, can be applied to the detection of trace organic molecules in water.