This work analyses the long-term trend of the daily global (GHI) and diffuse (DHI) irradiations received on a horizontal plane for four cities in China: Harbin, Beijing, Wuhan and Guangzhou, located from North to South. Measurements of GHI and DHI between 1990 and 2013 have been retrieved from GEBA and WRDC networks. During this period, the yearly mean of the GHI increases for most of the sites (0.1 to 0.7% per year) except for Harbin for which it decreases (-0.4% per year) while the yearly mean of the DHI increases for all sites (0.2 to 0.9% per year). The effects of the aerosol optical depth at 550 nm and the cloud cover on such changes have been investigated. It has been found that aerosols have a direct impact on GHI in clear-sky conditions, especially for Beijing and Wuhan, and that the correlation is strong between the GHI measurements for all-sky conditions and aerosol optical depth at 550 nm. Expectedly, the correlation is much more significant between the GHI measurements and the cloud cover.