In order to improve the precision of demodulation phase of coupler in optical fiber pickup system, a 3×3 coupler phase demodulation algorithm is proposed in this paper. Based on the demodulation of 3×3 coupler, this algorithm firstly compensates the direct current component of two interference signals by iterative operation, and then demodulates the audio signal. The experimental results of audio measurement show that this algorithm can effectively restore audio signal and it is practical.