To determine the emissivity of the primary mirror in a space camera and to analyze the thermal control effort for a subsystem, a method for measuring emissivity by using two kinds of materials with known emissivity as references was proposed based on the analysis of temperature measurement principle of a thermal imager. According to the emissivity measurement experiment, the primary mirror emissivity was calculated to be 0.565. To determine the influence of different measuring factors on the measurement accuracy, an error analysis was carried out. Analysis results prove that when the materials to be measured have almost the same emissivity with the objects, the temperature measurement errors of the thermal imager and emissivity standard error of the material will have greater impacts on the measurement accuracy. The measurement error caused by temperature measurement error and emissivity error is ±0.028 in the proposed experiment. Finally, the thermal model of the camera was modified by using a thermal equilibrium test, the results demonstrate that the emissivity measured by proposed method reflects the actual state of the primary mirror, and the method is useful and feasibility for measuring the emissivity of primary mirrors.