The solar collectors were exposed to the ambient conditions and left without cleaning for three months in winter in Lanzhou. In this paper, the effect of dust accumulation on the instantaneous efficiency, stagnation temperature and temperature rise of flat plate solar collectors is analyzed. The efficiency of solar collectors with and without dust in steady-state condition is measured following the Chinese National Standard GB/T 4271. Beside this, the stagnation test is carried out. The results show that the optical efficiency decrease of 9.7% and the stagnation temperature decrease of 0.3 °C–7.2 °C over time in a solar collector exposed for three months without cleaning. Furthermore, daily test is conducted. The efficiency, dust shading coefficient, and temperature rise of solar collectors are calculated by using experimental data. In this experiment, the dust shading coefficient changes between 0.83 and 0.94 during the whole day, and it is lower at noon than in the morning and evening hours. The efficiency difference and the temperature rise difference between the solar collectors with and without dust also vary with the dust shading coefficient, the optical efficiency, total solar irradiance, and mass flow rate during the whole day.