Abstract: In order to study laws of soil and water loss of tropical rainforest and the exploitation and utilization of its land.We have carried out the field-plotexperiment of water loss and soil erosion in southern Yunnan for six years from 1962 to 1967. The results of the study are summarized as follows: 1. The seasonal variation of soil and water loss in southern Yunnan is very obvious,it may be divided into three different periods on the basis of soil and water loss during six years. (1) The period of slight soil and "water loss in hot and dry season (Feb.to May). (2) The period of severe soil and water loss in rainy season(Jun.to Aug.). (3) The period of slight soil and water loss in cold and cool season(Sep.to Jan.of next year). 2. The general law is that the runoff,the soil erosion and the runoff coefficient increase with every increase of rain capacity and rainfall intensity in this region. 3. In different exploitation,the soil and water loss of the rubber-tea community is 22.3% and 29.7% 1ower than that of a pure rubber plantation, and 95.7% and 93.9% lower than that of farm crop.Therefore,to transform farm land into rubber-tea community or economic forest land of two stories is a better way to utilize tropical mountainous area rationally and to decrease loss of soil and water. 4. The results of the study still shows that the loss of soil and water on the farm land of shifting cultivation is the most serious.It is 778.3 times and 34.5 times that of tropical rainforest,and 6.6 times and 5.7 times that of bench terrace respectively.In order to restore the function of tropical rainforest ecosystem and to prevent the loss of soil and water on farm land in this region,wé must safeguard tropical rainforest against deforestation.We must change of the slope land to contour terrace,and reform step by step the cropping system of shifting cultivation into that of rational rotation of crops,these are efficient measures to raise soil fertility,keep water and soil, fix farm land and obtain high yield of crops.