The invention discloses a method for measuring a multi-mode relaxation process of a mixed gas molecule. The method comprises the following steps: establishing an acoustic spectrum expression of a single relaxation; when omega tau=1, acquiring a general expression form of the single relaxation process of the acoustic spectrum, thereby acquiring an expression form of valid heat capacity of the multi-mode relaxation process; measuring a sound absorption coefficient alpha r(omega) and a sound velocity c(omega) of 2*N frequency points, thereby acquiring 2*N non-dimensional sound relaxation absorption coefficient mu m(omega); and solving N epsilon i and tau i values from 2*N equations, substituting N epsilon i and tau i values into an original equation, thereby acquiring a spectral line of the mu m(omega) about omega=2 pi f, namely, reconstructing the multi-mode relaxation process. The method for measuring the multi-mode relaxation process of the mixed gas molecule, disclosed by the invention, not only is simple in operation but also is capable of accurately reconstructing a complete spectral line of the multi-mode relaxation process. An acoustic spectrum measuring method with higher adaptability and practicability is supplied for an accurate measuring technique for a gas sensor based on acoustic spectrum decomposition.