On-chip Fourier-transform spectrometers (FTSs) based on Mach–Zehnder interferometer (MZI) arrays suffer from severe central wavelength and fringe contrast variation due to fabrication errors. Even though a calibration matrix can be employed to correctly retrieve the input spectra, environmental temperature variation greatly degrades the retrieving performance. In this paper, we devise a dual-layer ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguide interferometer to reduce the temperature sensitivity. The beating of the even and odd supermodes in the dual-layer waveguide generates periodic intensity fluctuations in the spectrum. Since these two modes have similar modal profiles, their thermal sensitivity and propagation loss are relatively balanced, leading to a low temperature sensitivity and a high interference extinction ratio. We designed and fabricated a passive FTS based on a 32-channel dual-layer ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguide array. Experimental results show that the temperature sensitivity is reduced to 10 pm/°C, which is almost half that of single-layer ${\mathrm{Si}}_3{\mathrm{N}}_4$ MZI-based FTSs. With this chip, we accurately reconstructed various types of optical spectra, including single and two sparse laser lines, and broadband optical spectra. Our method can fit a wide wavelength range, which is a promising technology to improve the practical applications of on-chip FTSs.