Abstract: As one of the key design parameters for the fi xed grid connected solar PV array, the optimum tilted angle is not only related with the geographic location, but also directly affected by atmospheric conditions. On the basis of long-term variations of the observed solar radiation (global radiation, direct horizontal radiation, diffuse radiation, direct irradiation ratio and diffuse irradiation ratio) in Beijing, the variations of the optimum tilted angle for the fi xed grid connected solar PV array are calculated and analyzed in this paper. It is shown that: 1) there was a declining tendency in the global radiation during the recent 55 years, among various radiations, the decrease in the direct horizontal radiation took the main proportion while the decline in the diffuse radiation was not obvious, these lead to the significant decrease in the direct irradiation ratio while the obvious increase in the diffuse irradiation ratio. 2) The long-term optimum tilted angle variation also shows a downtrend. Compared with the optimum tilted angle in the 1960s, it falls by 2 degrees in the recent 5 years. It is presumable based on the calculation that the declining tendency in the optimum tilted angle is mainly caused by the significant decrease in the direct irradiation ratio, which is intensively affected by the atmospheric conditions. Therefore, the variations of atmospheric condition and its impacts on the solar radiation should be paid more attention for determining the optimum tilted angle in design and construction of the photovoltaic power plants.