A photonic method for instantaneous microwave frequency measurement based on stimulated Brillouin scattering was proposed. The unknown radio frequency signal is modulated by intensity modulator and then it is considered as pump light. The signals generated by vector network analyzer are modulated by phase modulator and then they are considered as sweeping signal light. When the phase matching condition is satisfied between pump light and sweeping signal light, the stimulated Brillouin scattering occurs and the conversion from phase modulation to intensity modulation selectively is achieved. The frequency of unknown radio frequency signal is ultimately measured. Experimental results demonstrate that the measurement ranges from 0.5 GHz to 27 GHz and the maximum measurement error is less than 20 MHz.