Abstract: The typical ravine rainforest and the mountainous rainforest in Xishuangbanna Nature Reserve in Mengla County were selected as objectives for diameter structure research.The results showed that the diameter distribution of the tropical rainforests was left bias before the stories were divided, and both the skew and kurtosis values were large.While the diameter distribution in each storey well followed the three -parameter Weibull distribution function, and the C storey also followed the negative index distribution function proposed by Meyer after the stories were divided, and both the skew and kurtosis coefficients were inversely proportioned to stand diameters.