A fiber sensor based on all polarization-maintaining fiber Mach-Zehnder interferometer is presented, in which the single-frequency polarization-maintaining optical fiber laser is used as a light source. An active feedback loop for laser source and a balanced-mixer receiver are adopted to suppress the noise of the sensor system. This system makes the intensity noise reduce about 15 dB, and the signal-to-noise ratio of the system is up to 60 dB.