Abstract: This study aiming at a contrasted observation with forest land and burnt land was carried out in the tropical semideciduous monsoon forest on the lower mountain of the western slop of the Jian Feng Ling mountain in Hainan Island. The area of the plot to observe the surface run-off is 100m2. The ecological consequence caused by "slash- and burn- cultivation" was investigated in the same year. After slash-and burn- cultivation of the forest :the microclimate became dry and hot rapidly and the relative humidity was decreased about 10--20%. The air temperature was increased 1.5-3℃, while the soil and the soil temperature was increased 8℃. The defoliation period of the forest trees was not obvious. As compared with the forest maintained, the mean run-off increased 5-6 times and the run-off coefficient was 4-5 times more. The soil lost was 105m3 per ha. and the amount was 7 times more than that on the forest land. The suspending silt was reaches to 362kg. the amount of which was 20 times more than those on the forest land. About 2cm topsoil was lost. The nutrients lost with the loss of water and soil. They include 19,800kg. of organic matters and 1,000kg. of total nitrogen, l60kg. of available potassium, l6kg. of available phosphorus and 2000kg. of exchangeable base. However, the soil acidity is not increased significantly. The tropical forest ecosystem plays a great role in environmental protection. The “slash-and burn-cultivation” is very destructive. Therefore, this way of land-use should be abolished.