This paper presents a new method for measuring the half-wave voltage $V_{pi}$ of an electro-optic phase modulator based on a phase-modulated photonic link with interferometric demodulation. By using this method, the $V_{pi}$ can be obtained with the RF voltage amplitude input required to achieve 1-dB gain compression of link and the differential delay of a Mach-Zehnder interferometer. We measure the $V_{pi}$ of a commercial phase modulator by using the presented method and the carrier/the first sideband intensity ratio method. Furthermore, we compare the two measurements with the typical value provided by the manufacturer. The experiment shows that this novel measurement method is feasible, straightforward, and accurate.