A microwave instantaneous frequency measurement scheme based on single lightpath polarization multiplexing technology was proposed. This technique requires only a dual-polarization dual-drive Mach-Zehnder modulator to provide the pump light and probe light needed for stimulated Brillouin scattering. Compared with the previous dual-channel Brillouin scattering technique, the proposed technique offers the advantages of a simple structure, compression system volume, stable and controllable pump light and detection light interference, and improved system stability. The unknown microwave signal was modulated using carrier suppression double sideband modulation as the pump light, and the swept-frequency detection signal was phase modulated as the scanning detection light. Moreover, the conversion from phase modulation to intensity modulation was realized using the stimulated Brillouin scattering effect. The microwave signal instantaneous frequency was measured by establishing a mapping relationship between the scanning frequency and the output optical power. Theoretical and simulation models were established to study the influence of the pump light wavelength jitter, DC bias point drift, phase drift of electric phase shifter, and polarization state shift of pump light and probe light on frequency measurement accuracy. Research results show that the proposed scheme can measure the frequency of microwave signals above 30 GHz. Furthermore, the maximum absolute measurement error does not exceed 30 MHz, while the relative measurement error is less than 2%. The frequency measurement range can be increased by increasing the scanning range of the frequency sweep detection signal and the modulation frequency range of the modulator. The proposed scheme has the advantages of a compact structure, stable performance, and simple operation and has a wide application prospect in low-cost and wide-spectrum radar detection.