The relationship between the emissivity of millimeter wave stealthy coat, the real temperature of target surface and some typical environment emissivity was calculated; based on that, the emissivity value range expression of three color millimeter wave stealthy coat was deduced with the typical environment emissivity. The relationship between the color block emissivity value ranges with the real temperature of target surface was discussed in three different cases. The result shows that in the 300-400 K, the main color emissivity value should be similar to the environmental emissivity(for the ground, εg=0.935), the harmonize emissivity value changes monotonically with the target surface real temperature; they have the same value range, and decrease with the increase of the target surface real temperature. The conclusion plays a guiding role in designing the millimeter wave stealthy coat.