Abstract: The variation of global solar radiation reaching the surface over the Wuxi region and its calculation are studied for further understanding of the solar transfer on water-air interface and the underwater irradiance. The main methods and formulas for climatological calculation of global radiation are simply discussed. Based on the observation data of global radiation and relative sunshine at Shanhai, Nanjing, Hangzhou stations, the empirical coefficients for these stations are calculated by using the least square method. The coefficient for the Wuxi region is calculated through interpolation from the coefficients of the three stations. The climatological calculation formula, which is suitable for Wuxi region, is brought forward. The global radiation reaching the surface is gained by using the formula and relative sunshine data at Wuxi station in recent 40 years. The formula precision is checked up by use of the observation data from the Taihu Lake ecosystem research station, which is proved to credible. At last, by analyzing the calculation results, the characteristics and causes of global radiation variation are discussed. The results show that the global radiation has decreased in the past 40 years and the decrease mainly attributed to the increase of suspended matter in the atmosphere. The annual variation of global radiation is basically similar to extraterrestrial irradiation except some differences in summer. These differences are caused by Meiyu with greater precipitation, cloud cover and less sunshine.