According to the analysis on the system structure of the microwave instantaneous frequency measurement based on phase modulation, detailed structure simulation and analysis of device parameters are performed on Optisystem software. With different input carrier wavelengths, section measurement results are obtained. Within the measurement range of 6~18 GHz, the lowest error is 0.1 GHz. The unknown frequency is got from detecting the ratio of output power, and measurement errors of about 0.5 GHz, 0.2 GHz and 0.1 GHz are realized for the measurement ranges of 6~11 GHz, 11~15 GHz and 15~18 GHz, respectively.