Abstract: Microwave blackbodies provide high-precision brightness temperature signals for microwave radiometers to accurately calibrate observed target radiation signals. The emissivity of a microwave blackbody is a crucial parameter affecting its radiative characteristics. Therefore, accurately measuring blackbody emissivity is significant for enhancing radiometer calibration precision and ensuring measurement value traceability and effective transfer. Currently, blackbody emissivity is mainly obtained indirectly by measuring reflectivity. This study implements two calibration methods in free-space monostatic reflection coefficient measurement: the offset-short calibration method and the sliding-load calibration method. Time-domain gating techniques are utilized to address multipath reflection signals during small reflection measurements. A reflectivity measurement system was established, and the reflectivity of the same blackbody target was measured within the 75-110 GHz frequency band, with results analyzed and compared. The error terms solved by the two calibration methods exhibit high consistency, with measured emissivity reaching levels of 0.999-0.9999. When the measurement target satisfies approximation conditions, the sliding-load calibration method proves more efficient. Finally, using the offset-short method as an example, the Monte Carlo method was employed to evaluate the uncertainty of the solved blackbody target reflection coefficient.