Solar reflectors are exposed to outdoor environments where dust accumulation is a primary degrading factor on optical performance. In this study, the effect of dust accumulation on reflectivity at different positions on the reflector of a parabolic trough solar thermal power plant in Hohhot, China was investigated and analyzed. The physical and chemical properties of dust accumulation were tested using a combination of spectrophotometer, scanning electron microscopy, and X-ray diffraction. The results showed that dust accumulation on the bottom edge of the reflector caused the largest decrease in reflectivity compared to dust on the center and top edge. In addition, dust particles at Hohhot were dominated by quartz (SiO2, 53.5%), followed by calcium oxide (CaCO3, 25.4%), and some minor feldspar minerals (NaAlSi3O8, 21.1%). However, some characteristics of the dust could not be determined by experimental measurements. To address this gap, a physical model was proposed to predict the impact of dust accumulation on light reflectivity of the reflector. Different physical parameters of the model are discussed, such as the size of the particles, diaphaneity, the incidence light angle, and tilt angle. The maximum relative deviation between the mathematical model and the experimental results was only 1%. (C) 2020 Elsevier Ltd. All rights reserved.