A novel multiple microwave frequency measurement scheme is proposed based on the selective conversion of phase modulation to amplitude modulation, which is realized by stimulated Brillouin scattering. The principle of this scheme is theoretically elucidated with its advantages and disadvantages discussed. Experiments are established to verify its feasibility and the results show that this scheme is competent in determining the frequency information among multiple frequency signal with very high resolution and accuracy. The maximum measurement error is less than 30 MHz.