In order to measure the frequency of microwave signal better, an instantaneous frequency measurement method based on phase modulation and intensity modulation was adopted. A continuous wave light source was divided into two channels by a coupler. Unknown microwave signals passed through both phase modulator and intensity modulator respectively. Accordingly, the carrier was modulated. Then it entered into two long-distance single-mode fibers. Because of the characteristics of microwave power loss caused by dispersion in optical fibers, the mapping relation between frequency and amplitude of monotone change was obtained. Then the amplitude comparison function was obtained by output power ratio of microwave signal detected by photoelectric method. In addition, the range and accuracy of frequency measurement were optimized. The results show that, the structure of the scheme is simple. It can measure the frequency of unknown signal quickly and accurately. The measurement range can reach 0.5GHz~53GHz. The measurement error is less than ±200MHz. This method can effectively measure the frequency of microwave signal and has strong reliability and wide application range.