For the distributed fiber optical sen sor based on the principle of mede-coupling of polarization-maintaining fiber and the theory of partial coherence of quasi-monochromatic light,it is of great importance to process the weak optical signal from the sensing fiber.The characteristics of the optical slgnal was analyzed and suggestions were given for medified Mach-Zehnder interfer-ometer,which in cludes an acoustooptical medulator and a polarized-beam splitter to realize the het-erodyne in terferen ce. The method not only eliminates the influences of light source fluctuation and background light,but decreases the loss of the signal ptocessing,and improves S/N ratio.The experi-ment results coincide well with the analysis.