A new method is proposed to measure the modulation coefficient of the optical phase modulator based on piezoelectric ceramic (PZT). The modulation coefficient of the optical phase modulator based on PZT can be calculated by 3×3 demodulation algorithm, which utilizes the functional relationship between the output optical intensity and the phase difference between the two arms of the Mach-Zehnder (M-Z) interferometer. A second phase modulator is placed on another arm of the M-Z interferometer to decrease the influence of the system noise, and used as the interference source for the next experiment. We finally use the phase generated carrier (PGC) algorithm to demodulate an interfered signal of the M-Z interferometer to test the result. The whole scheme is successfully demonstrated both by numerical simulation and experimental results.