A simple but reliable method, namely the working-point control by tuning the laser frequency, for the dynamic phase shift measurement in a passive homodyne interferometric fiber-optic sensor is proposed. A dc voltage calculated from the photodetector output is applied to the light source to control the interferometer at the condition of maximum sensitivity. Then the signal’s phase shift can be obtained from the components of zero and fundamental frequencies. To test the method, an all polarization-maintaining Mach–Zehnder interferometer with a piezoelectric ceramic ( $\mathrm{P}\mathrm{b}\mathrm{Z}\mathrm{r}\mathrm{T}\mathrm{i}{\mathrm{O}}_3$ , or PZT) cylinder in one arm is constructed. The experimental results show that the simulation signal’s phase shift generated by the PZT cylinder can be read out correctly with the method. It has the advantages of simplicities of operation, no-active element in the sensing head, and large operating bandwidth. It can be used for readout of dynamic phase shifts in various interferometric fiber-optic sensors.