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Fiber optic sensors have been used widely during the past two decades. Demodulation module is very important in the system. The performance of the system may be suffered significantly if the demodulation algorithm is not properly carried out. The demodulation algorithm for a Mach-Zehnder interferometric fiber acoustic sensor with 3times3 coupler is investigated in this paper. The mathematical expression of the demodulation error is given under the condition that the splitting ratio of the three arms is not even. Based on that, a method to rectify the unbalance is proposed, which shows good performance in simulations.