The invention discloses a data demodulation method for an optical fiber sensing system based on a heterodyne scheme, wherein an initial sampling rate is N times the heterodyne frequency, the down sampling multiple M is an integer multiple of N, and the number of elements in the low pass filter coefficient is an integer multiple of the down sampling multiple M. according to the method in the invention, the filter coefficient is reasonably designed together with the appropriate down sampling multiple to complete the multiplied carrier operation and low-pass filtering operation, which are originally separately carried out, in one step, thus reducing the computational load of the optical fiber sensing system based on heterodyne demodulation, and ensuring the continuity between two adjacent sampling time strongpoints in the two down-sampled orthogonal coefficients.