A novel, highly sensitive dual-parameter sensor based on a composite interference structure has been designed and implemented in this paper. The sensor was composed of a Fabry–Perot interferometer, where the microcavity was filled by magnetic fluid (MF) and a Mach–Zehnder interferometer, where the photonic crystal fiber was filled with ethanol. The refractive index of the MF and ethanol would be changed with the increase of magnetic field and temperature. Hence, simultaneous measurement of temperature and refractive index can be achieved by monitoring the reflected spectrum and transmitted spectrum. The sensitivities of MF and ethanol can reach up to 0.033 nm/Gs and 0.236 nm/°C, respectively.