The paper presents the modeling to measure the emissivity of microwave blackbody. Different from the computation and measurement of general plane absorbing material,the emissivity measurement of microwave blackbody need consider near field effect,scattering effect and influence coming from measurement system reflection. Hence the measurement modeling of the former can not work well at measuring the emissivity of the latter. The modeling presented in this paper gives the process of how to compensate the near field effect and the scattering effect and how to separate the measurement system reflection and the microwave blackbody reflection by using space method with moving DUT. To explain the modeling clearly，the configuration design of microwave blackbody,emissivity measurement system，measurement process and original data of 3mm microwave blackbody are also presented in the paper.