According to the Stefan-Boltzmann’s law, infrared (IR) stealth can be realized by reducing both the surface temperature and the surface IR emissivity. In the practical applications of IR stealth coatings, in order to reduce the surface temperature of high-power military equipment, a thermal protection layer is often stacked between the particle-composite low emissivity coating and the bare equipment skin surface. However, the radiation transfer process in the stacked structure will differ from the case of only particle-composite low emissivity coating used, hence the apparent emissivity of the stacked IR stealth coatings will deviate from the originally designed particle-composite low emissivity coatings, which will further affect the IR stealth performance. Therefore, it is of great engineering importance to investigate the effects of the material and thickness of the thermal protection layers on the apparent emissivity of the stacked particle-composite IR stealth coatings.A structure model composed of particle-composite low emissivity coating, thermal protection layer and substrate is built (Fig.1). The filler particles are simplified to be spherical and distributed uniformly in the top particle-composite low emissivity coating layer. The Finite-Difference Time-Domain (FDTD) method is applied to simulate the electromagnetic response of the stacked structure in the spectral range of 3-14 μm. The spectral average emissivity of the stacked structure is extracted to investigate the effects of the thickness of the thermal protection layer of different material (YSZ, SiO₂ aerogel, and other materials of different complex refractive index). Moreover, the power absorption density distribution is calculated to analyze the mechanism of the thermal protection layer on the apparent emissivity. The apparent spectral emissivity of the IR stealth coating is increased after a YSZ or SiO₂ aerogel thermal protection layer is added between the particle-composite layer and the bare skin surface (Fig.5). The spectral average emissivity within the long-wavelength IR detecting range of 8-14 μm increases from 0.27 to 0.61 and 0.49 when the thickness of YSZ and SiO₂ aerogel thermal protection layer increases from 0 to 20 μm respectively (Fig.6), while the variation within the long-wavelength IR detecting range of 3-5 μm is relatively low, which indicates that the IR stealth coating has begun to lose its low emission characteristics. The sensitivity of the emissivity to the thermal protection layer thickness is rarely affected by the embedded-particle size and volume fraction in the top particle-composite layer (Fig.7, Fig.8), but mainly depends on the intrinsic radiative properties (the complex refractive index) of the thermal protection layer. The variation of the apparent emissivity with the thickness of the thermal protection layer of different complex refractive index shows that the sensitivity of the emissivity to the thermal protection layer thickness is mainly dominated by the imaginary part of the complex refractive index, namely the absorption coefficient of the thermal protection layer. The higher the absorption coefficient of the thermal protection layer, the more sensitive the apparent emissivity of the stacked IR stealth coating is to the thermal protection layer thickness (Fig.10). The underlying mechanism of the apparent emissivity variation with the thermal protection layer thickness can be proved by the Kirchhoff’s law and the power absorption density distribution in the thermal protection layer: 1) more strong absorption zones will be included as the thermal protection layer thickness increases, which will lead to more absorption of the incident energy (Fig.11); 2) more energy will be absorbed by the thermal protection layer with high absorption coefficient after the incident light is transmitted through the particle-composite layer (Fig.12).The YSZ or SiO₂ aerogel thermal protection layer stacked between the particle-composite low emissivity coating and the bare equipment skin surface will increase the apparent emissivity of the IR stealth coating, and the apparent emissivity increases with the increase of the thermal protection layer thickness. The intrinsic absorption coefficient of the thermal protection layer is the dominant factor for the sensitivity of the apparent emissivity of the stacked IR stealth coating to the thermal protection layer thickness. The higher absorption coefficient, the more sensitive the apparent emissivity is to the thermal protection layer thickness, which may cause the IR stealth coating lose its low emissivity characteristics. Therefore, in the design and applications of particle-composite IR stealth coating, the improvement of the stealth performance by stacking thermal protection layers should be comprehensively evaluated from both increasing thermal conductive resistance and suppressing the apparent IR emissivity, the material and thickness of the thermal protection layer should be selected carefully and reasonably.