A broadband tunable instantaneous frequency measurement (IFM) system is designed based on the stimulated Brillouin scattering effect of the highly nonlinear fiber in which the carrier suppressed single sideband modulated signal of the Brillouin frequency shift acts as pump light. The amplitude comparison function (ACF) is constructed by the power radio of the two paths in the system. The frequency measurement range and measurement accuracy can be tuned by changing the frequency difference of the two phase modulation signals. The tunable frequency measurement ranges of 2-5 GHz, 2-10 GHz, 2-15 GHz, 2-20 GHz, and 2-24 GHz are realized, and the corresponding measurement accuracies are 3.64 dB/GHz, 2.17 dB/GHz, 1.87 dB/GHz, 1.22 dB/GHz, and 0.77 dB/GHz, respectively.