A Fourier analysis applied to the Mach–Zehnder interferometer (MZI) transmission spectrum for simultaneous refractive index (RI) and temperature measurements is proposed and experimentally demonstrated in this Letter. In the fast Fourier transform (FFT) spectrum of the MZI transmission spectrum, several frequency components are generally observed, which means that the transmission spectrum of the MZI is formed by the superposition of some dual-mode interference (DMI) spectra, and each frequency component represents different core-cladding interferences. We can select some dominant frequency components in the FFT spectrum of the MZI transmission spectrum to take the inverse FFT (IFFT). Then, the corresponding DMI patterns can be obtained. Due to the shift of the wavelength of these DMI spectra with changes in the environmental parameters, we can use the coefficient matrix of these DMI spectra for multi-parameter sensing. In this Letter, two DMI patterns are separated from the resultant transmission spectrum of the MZI. As the RI and temperature change, the shifts of the two DMI patterns with respect to the RI and temperature will be observed. The sensitivities of the RI and temperature are $137.1806\mathrm{nm}/\mathrm{RIU}$ (RI unit) and $0.0860\mathrm{nm}/\mathrm{°C}$, and $22.9955\mathrm{nm}/\mathrm{RIU}$ and $0.0610\mathrm{nm}/\mathrm{°C}$ for the two DMIs. Accordingly, it can be used to simultaneously measure RI and temperature changes. The approach can eliminate the influence of multiple interferences and improve the accuracy of the sensor.