The changes in terrestrial forest vegetaton at different historic periods are an important factor related to climate change. With forest thickness as the indicator to measure the change in forest vegetation, the paper studied the quantitative relationship between forest changes and precipitation in the past 4000 years in China by historical data deduction, theoretical derivation and statistical analyses. The results showed that in the past 4000 years the average annual precipitation in China had decreased from 918mm to 612mm in the 1970s-1980s，and then increased to 628mm in the early 21st century, while the forest thickness had decreased from 9.38mm to 1.03mm in the 1970s，and then rose to 1.58 mm at the beginning of the 21st century. As forests have a stronger evapotranspiration capacity compared with other land uses such as farmland, grassland and urban land，and China is located in the area under the special southeast monsoon climate at that, it may be concluded that the forests in China have imposed great impacted on precipitation change. Generally, every 1mm change in forest thickness can lead to 37mm change in precipitation. In such sense, the frequent droughts in the 1970s-1990s did not occur by chance, e.g. Luobu Lake dried up, the historically lowest water level appeared at Qinhai Lake, the Yellow River has saw zero water flow for 22 times, more severe land desertification, etc. At a large scale, these drought can be regarded as necessary results since the extreme forest destruction in the past 4000 years has triggered the climate change and finally the aridification. The heterogeneity of deforestation in temporal and spatial dimension in history may lead to the phase-based occurrence and mutation of climate change. Between the years of 280 DC-1230 DC, the subtropical and tropical forests in the southern part of China had suffered from the large-scale destruction as a result of population migration to the south and demographic growth, and then there appeared a continuous aridification. This example demonstrates that much more attention should be paid to the impact of forests in southern China on climate change in the whole country, especially in the northern part. Aridification has resulted from the decline of forest vegetation, while it is a major factor to worsen land desertification in China. Therefore, to combat the desertification in northern China, it is necessary to vigorously increase forest resources in terms of both the forest coverage and the forest thickness. It is expected that in the future, as long as the forest thickness increases, the average precipitation per year will rise, the desertification in China will be controlled, and the production potential of large-area land in northwestern China will be tapped and developed gradually.