Abstract: Using data from permanent sample plots, this paper studied the dynamics of diameter structure of Chinese fir plantations and affection of density on it, some criterions was adopted such as skewness, kurtosis, variance coefficient, cumulative diameter distribution curves and frequency distribution and so on. The results show:1.In the age range of 6～20,the value of skew changes from negative to positive, the absolute value of skew first gets small,then change to big, at any time, high density stands have the bigger skew, the higher the density of stands is, the earlier the skew varies from negtive to positive;2.The variance law of kurtosis with age is not obvious,kurtosis of stands with low density is bigger than stands with high density, and the value of kurtosis,both of them,tends to 0 at last.3.The diameter variance coefficient shows weakly increasing trendency with age on the whole, and declines in earlier stage,then increases after canopy closure. non-evenness quality of high density stands is relatively big,and the bigger the density is, the earlier the variance coefficient of diameter shows increasing trendency.4.At any range of cumulative frequency distribution,the more smaller the density is, the bigger the middle value of diameter class is, which benefits the formatioin of middle and big wood.5.The size-class distribution testifies these conclusions.