Phase-sensitive optical time domain reflectometry (φ-OTDR) uses narrow line-width laser with high coherence. In the sensing fiber, scattered lights from optical pulse interfere, and the interference light is highly sensitive to external disturbance. But the traditional amplitude-detection φ-OTDR is not quantitative detection, so that phase demodulation is necessary. Phase demodulation system based on 3×3 Michelson interferometer is cost-effective, but is of lower spatial resolution than the traditional amplitude-detection φ-OTDR. We propose a four-channel detection method to preserve the same spatial resolution as amplitude-detection φ-OTDR by adding only one detection channel. But the interference of scattered lights may have critical influence to phase demodulation. A four-channel detection system of phase demodulation φ-OTDR is analyzed based on 3×3 Michelson interferometer, from the aspect of Rayleigh backscattering light. The experiment shows that the disturbance around the fiber is detected quantitatively, with a linearity of 0.9956, phase sensing range of 31.85 rad, and the spatial resolution is same as that of amplitude-demodulation φ-OTDR.