A new measurement method for dynamic phase changes in fiber interferometers is demonstrated. The amplitude of the dynamic phase is obtained directly from the zero and fundamental frequency components of the fiber interferometer output by method of constructing match-functions without modulating the laser. It has the advantages of no active element in the interferometer, simple signal demodulation and large operating bandwidth. To test this new method, an all polarization-maintaining Mach-Zehnder fiber interferometer is constructed, and the simulation dynamic phase change induced by PZT is measured. Experimental results show that the calculated amplitude of the dynamic phase shift is a good linear function of the voltage applied to the PZT, which accords well with PZT’s properties of low-frequency and small amplitude. This method is applied to the measurement for the acoustic sensitivity of interferometric fiber-optic hydrophones, and the test results are in agreement with the results obtained by phase generated carrier method.