An instantaneous frequency measurement (IFM) system based on polarization time delay interference is proposed. The amplitude comparison function (ACF) is established by detecting the optical power of the two orthogonal polarization signals. The calculation results show that ACF is only related to the frequency of microwave signal to be measured and the time delay difference introduced by birefringence effect of polarization maintaining fiber (PMF), and the tunability of ACF measurement range and accuracy can be realized by adjusting the time delay difference. The simulation results show that the absolute error tolerance of the IFM system is 300 MHz in the frequency range from 0 to 25 GHz. Moreover, the effects of laser wavelength drift, radio frequency signal intensity, bias voltage shift of Mach-Zehnder modulator (MZM), MZM extinction ratio and polarization angle drift of polarization controller 2 (PC2) on the measurement frequency of the system are discussed. Compared with similar methods, the proposed scheme adopts the integrated structure of PMF and polarization beam splitting, which not only realizes the adjustable frequency range, but also simplifies the system structure and reduces the cost.