Aiming at the problem that the non-rotational symmetry of the three output signals of the 3×3 optical fiber coupler is easy to cause the error of phase demodulation, this paper puts forward the theory of the asymmetric optical fiber coupler phase demodulation system based on the two output signals of the 3×3 optical fiber coupler, and verifies the phase demodulation ability of the system for the single frequency periodic signal and the third harmonic signal through software simulation analysis. The phase demodulation performance of the optical fiber vibration sensing system is tested on the phase-sensitive optical time domain reflectometer ( Φ-OTDR). The experimental results show that the system can realize phase demodulation in the range of 200~ 4 000 Hz, the absolute error of vibration positioning is within ±10 m, and the spatial resolution of the system can reach 21 m. On the basis of reducing the hardware cost of the Φ-OTDR system, this method can provide a new solution and reference scheme for the problem of unsteady splitting ratio and phase difference of optical fiber coupler in Φ-OTDR system, and can be applied and popularized in the field of traffic safety monitoring