The polarization fading and phase noise of unbalance fiberMach-Zehnder interferometer is investigated in this paper. With the couple theory and the opticalwave-guide theory, the unbalanced fiberMach-Zehnder interferometer output expression, fringe visibility expression and phase noise expression are got. The relationship between the fringe visibility of interferometer, phase noise and polarization of input light is theoretically confirmed. Through theoretical analysis the method of restraining polarization fading and phase noise is put forward.