Chemical weathering in large-scale drainage basins has significant effects on cycling of sea and land matters and climatic change on Earth. Various sediment geochemical proxies have been proposed to study the influencing factors of chemical weathering intensity. However, quantitative description of chemical weathering intensity and knowledge of controlling factors of chemical weathering are still insufficient. Exemplified by the sediment geochemical data from the Yangtze River and Pearl River Basins, the chemical weathering intensity was quantified by Chemical Index of Alteration (CIA) and Weathering Index of Parker (WIP). In addition, the spatial distribution characteristics were studied by using Trend Surface Analysis (TSA), and the correlation of the chemical weathering intensity with temperature and precipitation was completed based on Spatial Correlation Analysis (SCA). The following results were obtained: \mathrm{①} The average values of the CIA and WIP in the Yangtze River Basin are 72.9 and 34.2, indicating an intermediate weathering intensity. The average values of the CIA and WIP in the Pearl River Basin are 93.8 and 6.4, indicating a strong weathering degree. \mathrm{②} The TSA shows that the distribution of chemical weathering intensity has obvious latitude effect, that is, the lower the latitude, the stronger the chemical weathering. \mathrm{③} The abnormal values of the CIA and WIP coincide with a higher annual temperature and precipitation in space. Meanwhile, the correlation coefficients of the CIA and WIP with annual precipitation and annual mean temperature are all above 0.85 in the study area. In conclusion, the chemical weathering in the Yangtze River and the Pearl River Basin is mainly controlled by climatic conditions. The CIA and WIP can be used as a quantitative indicator for the chemical weathering intensity in drainage basins, and the trend surface analysis well depicts the regional and local distribution characteristics.