The work aims to propose an in-situ measurement technology of radar reflectivity, which is based on the antenna near-field measurement, so as to meet the urgent requirements for in-situ measurement of radar reflectivity of microwave absorbing coatings under installation conditions. A miniaturized ultra-wideband antenna, a decoupling structure, and a shielding cover were designed, and an in-situ measurement probe was constructed. By integrating the probe with a microwave frequency scanning module and a data processing module, a handheld broadband radar reflectivity measuring instrument was developed. Through comparison and verification, the deviation of its measurement result from the arch system was less than 1.5 dB in the reflectivity range of −17 to 0 dB and the frequency range of 2 to 18 GHz. The instrument is suitable for accurate measurement of radar reflectivity of coatings with a thickness of less than 1.5 mm.