By using two phase modulators, two circulators, one electro-optic modulator, etc., stimulated Brillouin scattering can be generated in the upper and lower highly nonlinear optical fibers, forming an instantaneous microwave frequency measurement system for the bisymmetric stimulated Brillouin scattering effect. Using the one-to-one correspondence between the output optical power ratio of the system and the microwave signal to be measured, the amplitude comparison function is established to realize the microwave frequency measurement. Theoretical analysis results show that by adjusting the line center gain coefficient difference and the Brillouin frequency shift difference of the highly nonlinear fibers, the measurement range and measurement error of the microwave frequency can be changed. When the line center gain coefficient difference is 30 and the Brillouin frequency shift difference is 2 GHz, the frequency measurement range is 0?18 GHz, and the measurement error is less than 52 MHz. When the line center gain coefficient difference is 30, the Brillouin frequency shift difference is 0.4 GHz, the frequency measurement range is 0?21 GHz, and the measurement error is less than 148 MHz.