Different from that in the optical band, the blackbody in the microwave band is constructed in a coated cone array structure. The blackbody of this type can be used in calibrating microwave radiometers with standard brightness radiations, and needs to have a uniform surface thermal distribution and high emissivity. The emissivity study of such a blackbody can be performed based on the Kirchhoff's law of thermal equilibrium, in a reflection determination routine. The emissivity characteristics varying with frequency have been intensively studied, but their variations with direction and polarization have not received much attentions. Starting from the Floquet mode analysis, a reflection evaluation scheme for the blackbody is presented, which is more robust than that based on the back-ward RCS determination. Based on the presented scheme, the trends of emissivity varying with frequency, direction, polarization are studied, for a microwave blackbody design. Results show that the emissivity rises as the frequency rises in a range from X band to K band; and in the low frequency band, the trend of the vertical polarization emissivity varying with elevation angle is different from that of the horizontal polarization emissivity, and there exists an obvious phenomenon that the vertical polarization emissivity declines with the increase of elevation angle. These phenomena are related to the electromagnetic absorption characteristics of the coating layer.