A cadmium ion (Cd 2+ ) sensor based on polyvinyl alcohol (PVA)/tetraethyl orthosilicate (TEOS)/3-aminopropyl triethoxysilane (APTES) film integrated Mach–Zehnder interferometer was fabricated and evaluated. The three core fiber (TCF) is spliced with two single mode fibers (SMFs), one end of TCF and SMF are spliced with enlarged taper, and the other end of TCF is directly spliced with another SMF. PVA/TEOS/APTES composite film is coated on the surface of TCF. When the Cd 2+ is adsorbed by the sensing film, the relative refractive index of TCF cladding changes, the output interference trough wavelength shifts. By monitoring the wavelength shift information of the interference trough, the concentration of Cd 2+ will be obtained. In the concentration range of 0–0.4 μM, with the increase of Cd 2+ concentration, the interference trough shows a red shift. The sensitivity and the detection limit of the sensor are 7 nm/μM and 0.0143 μM (1.6 ppb), respectively. The sensor has excellent selectivity and good pH stability. It has the advantages of easy preparation, simple structure, and has potential application for the detection of trace Cd 2+ .