Based on the mathematical model of the microwave photonic link with ph ase modulation and interferometer demodulation,we propose a method to measure the half-wave voltage of a phase modulator.The half-wave voltage is calculated through the S21of the peak gain of the small signal with a vector network analyzer (VNA).The ex periment results show that the data measured by this method has a good agreement with the data provided by the manufacturer.By a p plying the measured data of the phase modulator to our actual microwave photonic link system (the length difference between the two arms is 0.02m),the simulation curve successfully fits well the experimental data.In our experiment s,the tuning fiber interferometer,which is used to convert the phase-modulated signal into intensity-modulated signal,is construc ted with two 3dB polarization maintaining couplers and a special fiber jumper.Additionally,this method has simple stru cture,easy operation and high practical value for measuring the half-wave voltage of the phase modulator.