As one of the main plantation management strategies, thinning plays an important role in improving biodiversity of forest ecosystem in China. However, the effects of different thinning intensities on undergrowth plant functional groups are still unclear. We investigated the changes of four different thinning intensities for eight years (slight thinning, ST, 34%; moderate thinning, MT, 50%; heavy thinning, HT, 74%; no thinning control, CK) on the understory vegetation and its related environmental factors in Cunninghamia lanceolata plantations in subtropical China. We found that the different thinning intensities had no significant influence on the grass species richness of plant functional groups (P>0.05). MT significantly increased weed species richness of plant functional groups (P<0.05), while HT significantly increased fern species richness of plant functional groups (P<0.05). ST significantly increased the species richness of woody plant functional groups (P<0.05). The ST, MT and HT could significantly increase vine species richness of plant functional groups (P< 0.05). In addition, the different thinning intensities had no significant effect on the important values of the functional groups of grass and woody plants (P>0.05), but HT significantly reduced the important values of the functional groups of weeds (P<0.05). Meanwhile, the ST significantly decreased the important values of the functional groups of ferns (P <0.05), and ST significantly increased the important values of the functional groups of vines (P<0.05). Principal component analysis (PCA) showed that the composition and distribution of understory plant functional groups of ST and HT were significantly different from those of the no thinning control groups. Overall, the interpretation rates of forest and soil factors on the variation of understory plant functional groups that examined by the variation partitioning analyses were similar. And the forest and soil factors could jointly determine the variations of understory plant functional groups in Cunninghamia lanceolata plantations.