Abstract: Two treatments of no selective thinning(CK) and selective thinning(T) were set up for 107 half-sib families of the second cycle of Chinese fir breeding population., selective thinning was carried out at the age of 31 a, the DBH of the stand age of 4～8,13,16,23,31～33 a was taken as the target trait, the mixed linear model method based on the best linear unbiased prediction was used to analyze the age trend of genetic parameters of DBH traits and the selection of excellent families. The results showed that during the age of 4～31 a, the phenotypic variation coefficient, additive genetic variation coefficient and individual narrow-sense heritability of DBH traits were 15.43 % ～22.02 %, 8.05 % ～14.03 %, and 0.201～0.549, respectively; the trend of variation coefficient of stand age decreased first and then increased, while the heritability continued to increase with stand age. The individual narrow-sense heritability of stand age 33 a and 32 a was 1.28 % and 9.47 % lower than that of 31 a, respectively. The genetic correlation between 4～16 a and 23 a was above 0.4, the genetic correlation among 31,32 and 33 years old was close to 1, the genetic correlation between 7 a and previous stand age and 31, 32, 33 a was below 0.4 and the genetic correlation was not significant. According to the selection rate of 10 %, compared with the excellent families selected in the rotation period of 23 a, the missing rate of selection in the primary stand age of 5～6 a, the selected stand age of 7～8 a, and the final stand age of 16 a was 63.6 %, 36.4 %, and 18.2 %, and the genetic gains were 3.53 %, 4.15 % and 6.23 %, respectively. The genetic variation and heritability of DBH traits of Chinese fir increased with stand age, and the genetic correlation between stand ages decreased with the increase of stand age interval. There was a significant genetic correlation between at young forest stage and rotation stage. Selective thinning increased the phenotypic variation, decreased the additive variation and decreased the heritability to some extent, the heritability decreased slightly, and the genetic control decreased slightly after selective thinning. Early selection can be carried out at stand age of 7～8 a, and final selection can be carried out at stand age of 16 a to ensure the accuracy of excellent family selection.