A method and system are for expanding a measuring range of a Mach-Zehnder sensor based on the calculation of optical length; the method includes: (1) performing calibration according to a known parameter to complete calibration of a Mach-Zehnder pressure sensor; and (2) for an unknown parameter, testing the unknown parameter first using the Mach-Zehnder sensor to acquire discrete data; processing the discrete data using a peak and valley synthesis algorithm to restore a diffraction order m; calculating an optical length value of the unknown parameter; and restoring, according to a calibrated relationship curve between the optical length and the parameter, the unknown parameter, thus expanding the measuring range of the Mach-Zehnder sensor to enable the Mach-Zehnder sensor to break through the limitation of the FSR and the spectral width of a light source. The measuring range can be theoretically expanded to infinitely great.