Abstract: China’s summer precipitation is primarily influenced by the East Asian summer monsoon, which is driven by land–sea thermal contrast. This study defines the European–Atlantic land–sea thermal contrasts and two types of East Asian–Pacific land–sea thermal contrast (East Asia–North Pacific and East Asia–Subtropical High), using ECMWF/ERA5 reanalysis data and CN05.1 gridded precipitation dataset from 1979 to 2022. The study examines the characteristics of the three land–sea thermal contrasts, along with the spatial and temporal patterns of summer precipitation in China. In addition, it analyzes the effects of land–sea thermal contrast on the mean and variability of summer precipitation, as well as the influence of variability on extreme precipitation. The results show the following: (1) The precipitation in Northeast China, the lower reaches of the Yangtze River, and the Qinghai–Xizang Plateau is significantly affected by land–sea thermal contrast. The mean and variability of summer precipitation in these regions are increasing, and greater variability raises the probability of extreme precipitation. (2) The land–sea thermal contrast between East Asia–Pacific and Europe–Atlantic has maintained an increasing trend over the past 40 years and continues to rise under the medium emission scenario, with land surface warming exceeding oceanic warming. (3) The primary driver of precipitation generation and change in Northeast China is the land–sea thermal contrast between East Asia–Pacific and East Asia–Subtropical High. Moreover, the key factor influencing precipitation in the lower reaches of the Yangtze River is the land–sea thermal contrast between East Asia and the Pacific. In the Qinghai–Xizang Plateau region, precipitation is mainly affected by the thermal contrast between the East Asia–Subtropical High and the Europe–Atlantic land–sea mechanism. Finally, LBM (Linear Baroclinic Model) simulation results further confirm that the Euro–Atlantic thermal contrast increases precipitation in the western Qinghai–Xizang Plateau but reduces precipitation in the eastern Qinghai–Xizang Plateau. These findings provide a crucial foundation for understanding the temporal and spatial variations of summer precipitation in China and the physical mechanisms behind them.