Abstract: In the calculation of solar energy resources, Ångström method, by means of the sunshine percentage is most commonly used. But it should be noted that this method is suitable for regions where the natural environment is relatively homogeneous. Such conditions are easy to get to meet for small countries or regions. However, for the large countries, e.g., China, it is inappropriate to make an assumption that the natural environment is homogeneous. On one hand, the impact of the terrain on radiation can't be ignored; on the other hand, other atmospheric factors that affect the radiation incident, such as the layer of atmospheric water content, content of aerosol particles and so on, also can't be ignored. Former Soviet Union is also a vast country, and its scholars exclude regions where the altitude is higher than 1500 m when they are dealing with the terrain effect. This approach doesn't resolve the radiation distribution in the mountain area, but from a scientific perspective, it is prudent and rigorous.In the previous researches, the selection of the basic radiation, such as extraterrestrial radiation, clear radiation has been paid more attentions to. This is very important for the Ångström method; however, it doesn't consider the former facts influencing solar radiation enough.The clear radiation close to the actual conditions is calculated based on SMARTS model from Gueymard. In SMARTS model, each factor influencing solar radiation is considered, such as the whole layer water content deduced by surface water content, the aerosol effect considered by horizontal visibility, and the ozone effect derived by ozone content distribution. The clear radiation calculated in this way can be used as basic radiation, which helps overcome the lack of previous work ignoring the atmospheric impact factor. Owing to lack of observation data, the clear radiation in the whole country is not calculated. Only the clear radiation at Chengdu Station and Emei Mount Station are calculated. With the previous method, the results are much the same for two stations, but with the new method, the differences are quite obvious.