Abstract: To evaluate the impact of dust accumulation on specular reflection characteristics and predict the trend of specular reflectance changes in dusty environments, the finite-difference time-domain (FDTD) method was employed to simulate the infrared reflectance of a specular surface under various dust accumulation conditions. Based on the response surface method (RSM), a multi-factor influence model for reflectivity was constructed, and the accuracy of the model was verified through bidirectional reflection distribution function (BRDF) measurement experiments. The results show that within the wavelength range of 1.55 μm to 5 μm, the specular reflectance increases with wavelength, decreases significantly with increasing dust density, and diminishes notably with increasing incident angle. The dust accumulation density, wavelength, and incident angle exhibit a significant synergistic effect on specular reflectance. For instance, at a dust accumulation density of 50 mg/m², when the incident angle is 55°, the relative increase in reflectance from 1.55 μm to 5 μm in wavelength is 7.5 times greater than that observed at an incident angle of 15°. Additionally, for a wavelength of 3.6 μm, when the dust accumulation density increases to 100 mg/m², the reduction in reflectance from an incident angle of 15° to 55° is three fold compared to the dust-free condition. These results can provide data support for stray light analysis in optical systems and offer guidance for the cleaning and maintenance of optical components such as mirrors.