The variation characteristics of stand layer index and the species diversity of understory shrubs in three age groups (juvenile, medium and near mature) of Cunninghamia lanceolata stands and their correlations were analyzed based on the observation of C. lanceolata stands in eighteen permanent plots in Fushou Forest Farm of Hunan Province. The results showed that there were significant differences in stand layer index among different age groups (P<0.05). The stand layer index of the three age groups ranged from 0.134 to 0.507, indicating that stand layer index was generally low, the tree’s vertical space was underutilized, and the differentiation between individuals was weak. As C. lanceolata trees grew, species richness, Simpson diversity index and Shannon index of understory shrubs increased first and then decreased. There were significant differences in richness between the juvenile and medium stands (P<0.05), while the Simpson evenness and Pielou index of understory shrubs varied insignificantly with the increase of tree age. The species richness index had a very significant positive correlation with the stand layer index, indicating that stand layer index was an important factor influencing the richness index of understory shrubs of C. lanceolata stands.