In imaging optical system, the relative humidity of the environment will affect the scattering characteristics of the optical mirror surface covered with polluted particles and further affect the performance of the system. According to the hygroscopicity, the polluted particles were divided into hydrophilic particles and hydrophobic particles. By using the light scattering model of pollution particles, the variation of the scattering efficiency factor of a single hydrophilic polluted particle on the relative humidity of the environment was analyzed. On this basis, combined with the statistical models of hydrophilic and hydrophobic pollution particles, the effect of the proportion of hydrophilic particles and the cleanliness level of the mirror surface on the scattering characteristics of the optical mirror surface with multi-particles was analyzed. The results show that the bidirectional reflection distribution function (BRDF) and the total integrated scattering (TIS) caused by the polluted particles on the optical mirror surface increase with the increasing of the relative humidity of the environment. The higher the proportion of hydrophilic particles is, the stronger the scattering loss of the optical mirror surface is. As the cleanliness level of the optical mirror surface decreases, the influence of the ambient humidity on the scattering loss of the mirror surface decreases.