In order to improve the working efficiency of Mach-Zehnder interference fiber optic sensing technology applied to the field of underground urban medium and low voltagecable anti-external force damage warning, it is necessary to clarify the relationship between the interference voltage signal and its vibration force. Firstly, the paper theoretically analyzes the relationship between the interference voltage and the vibration force, and points out that when the external environment is certain, the rate of change of the interference voltage is only related to the vibration force. On this basis, the design experiment is carried out to study the relationship between the vibration wave propagated on the rigid cement surface and the interference voltage signal, and the propagation and attenuation laws of the vibration wave and the interference voltage signal are compared and analyzed. The results show that the peak value of Mach-Zehnder′s interferometric voltage change has a positive correlation with the magnitude of the vibration force. The peak value of the voltage change speed can be used as the feature quantity to predict the distance or strength of the intrusive external force from the underground cable.