In modern radar systems and electronic warfare systems, instantaneous microwave frequency measurement (IFM) is widely used for detecting and roughly classifying unknown signals. However, conventional electrical approaches realizing IFM have hit the bottleneck of limitation in measurement range due to the limited bandwidth of the electronic components. Photonics-based approaches for microwave spectrum analysis are considered to be competitive alternatives because of the advantages such as wide instantaneous bandwidth, low loss and immunity to electromagnetic interference. In the past decades, a few methods of photonic approaches have been proposed. A tunable fiber Fabry-Perot interferometer (FFP)<sup>1</sup> and a fiber Bragg grating<sup>2</sup> used as an optical scanning receiver were reported, but the response time is long due to the piezoelectric ceramics (PZT) or electric heating driven systems, the scanning speed is only 200 Hz 2. IFM based on frequency-amplitude mapping technique was previously demonstrated.<sup>3, 4</sup> However, the measurement range is limited to about 20 GHz and the accuracy varies in the whole range. In recent years, frequency measurement based on stimulated Brillouin scattering (SBS) with high resolution was reported,<sup>5, 6</sup> but the existence of a scanning microwave signal source made the system complex and the response time is depended on the sweeping speed of the local oscillator, which is of the order of milliseconds. In Ref. 7, a system integrating SBS and a frequency shifting recirculating delay line (FS-RDL) was demonstrated. The sweeping time is about hundreds of microseconds, but there is a trade-off between sweeping time and measurement range. Measurement period of 5 s in 20 GHz range has been realized by channelized radio frequency measurement scheme.<sup>8</sup> However, the implementation of an analog-to-digital converter with bandwidth of 2 GHz made the system complex and costly. <p> </p>In this work, a novel approach of ultrafast frequency measurement based on electro-optic Fabry-Perot (EOFP) scanning receiver is proposed for the first time. In comparison with other frequency scanning measurement systems, which use methods such as mechanical tuning, electric heating, scanning microwave signal, FS-RDL, etc., our double-EOFP system can measure signals with frequency under 54 GHz in 2 s, which is the fastest scanning rate as far as we know.