Abstract：Rainfall is known as the main factor which affects the stability of slope. With the development of unsaturated soil mechanics，a new way is coming to solve how suction and temporary water pressure affect the stability of slope. During the period of rainfall，water in the slope will change，at the same time the suction and the temporary water pressure will also change with time and rainfall. In order to know the characteristics of the change，the experiments to observe the change of suction and temporary water pressure are conducted. An observation well is built on the Xietan slope of the Three Gorges of the Yangtze River in May 2002. The well has a depth of twenty meters and a diameter of two meters. It is the largest well of observation on suction in the world. The well is put into action；and the data of the suction，temporary water pressure and rainfall is observed in October 2002. The work of observation is finished by the end of 2004. At last，by analyzing a large number of data and records，the character and change feature of the suction and temporary water pressure are found. The conclusions that the distribution of suction is decided by the materials and structure of slope are proved. The change of suction is related to the factors such as rainfall，time，depth，season and so on. The suction doesn¢t change promptly after the rain；there is time difference between the suction change and rainfall. It is obvious that these conclusions will guide to make further studies on how rainfall affect slope stability.
