The invention discloses a spectrum demodulation method and demodulation device based on a vernier effect. The demodulation method includes the steps of measuring a vernier effect spectrum of a cascadestructure, performing fast Fourier transform on the spectrum, an obtained frequency spectrum having three clusters of frequency components, selecting a spatial frequency component with a minimum frequency value, extracting the spatial frequency through a Gaussian filter, and performing fast Fourier transform inversion on the extracted spatial frequency to obtain an envelope of a cascade vernier spectrum. A vernier effect is formed through cascade reference of In-line MZI and the envelope of the vernier spectrum is extracted, and under the same condition, the drift amount of the cascade vernier spectrum envelope is about FSR(R)/(FSR-FSR) times of that of a sensing In-line MZI spectrum, thereby realizing amplification of sensitivity.