The tropical rainforest in Xishuangbanna, Yunnan Province of China, is introduced in detail in this paper. Situated at the northern margin of tropical mainland SE Asia and controlled by monsoon climate, the region has been climatically at the lower limits for tropical rainforests, however true tropical rainforests exist and develop luxuriantly in the region. The reasons for this are discussed. In general phytocoenological characteristics such as vertical stratification, life form spectrum, species riches etc. the tropical rainforest in Xishuangbanna is very similar to the typical tropical rainforest in equatorial region, but it is characterized by a clear change of physiognomy between different season. As occurred at the latitudinal and altitudinal limits of tropical rainforest, the flora of the rainforest is endowed with the nature of northern margin of tropical zone of SE Asia and is transitional toward the flora of subtropical forest of China. In recent years the region has been opened up to use in a large scale and the primary forests, especially rainforests, have been severely destroyed. The conservation and research to the tropical rainforest are very urgent and have to be done at once.