This paper proposed a copper-ion sensor with a Mach-Zehnder interferometer structure based on an ion-imprinted composite film. A section of single-mode fibers (SMFs) was spliced between two sections of three-core fibers (TCFs) and another two sections of SMFs were coupled to the ends of the two sections of the TCFs to form a TCF-SMF-TCF sensing structure. A chitosan (CS)/polyvinyl alcohol (PVA) composite film, a copper-ion-imprinted composite film, and an optimized copper-ion-imprinted composite film were coated on the surface of the TCFs, respectively. The copper-ion concentration was accurately detected according to changes in the relative refractive index of the TCF cladding caused by the specific copper-ion adsorption of the composite films. The experimental results demonstrate that as the copper-ion concentration increases, the monitored troughs in the transmission spectra of the fiber-optic sensors coated with the CS/PVA composite film, copper-ion-imprinted film, and optimized copper-ion-imprinted film exhibit red shift, red shift, and blue shift, respectively. A comparative analysis shows that the sensor coated with the optimized copper-ion-imprinted film displays the optimal response to copper ions. It has a response sensitivity of 62.258 pm·μmol ^-1·L and a detection limit of approximately 0.602 μmol·L ^-1, with good selectivity and high pH stability. This sensor, with the advantages of easy preparation and simple structure, has application potential for high-selectivity copper-ion detection in water.