In order to improve the forest quality of Cunninghamia lanceolata plantation, ecological thinning experiment was carried out to explore the effects of thinning intensity on understory plants, biomass and environmental factors, and to provide theoretical basis for improving the stability of Cunninghamia lanceolata plantation and maintaining its sustainable management. Cunninghamia lanceolata plantation in Jiangxi Guanshan Forest State Farm was used for the experiment.One-way analysis of variance, Duncan range method and Pearson correlation analysis were used. The impacts of different thinning intensities (0%, 20%, 40%) on the understory species composition, species diversity, above-ground biomass, stand light environment and soil chemical properties were investigated, as well as the response of species diversity to each influencing factor. The results indicated that:(1) The number of species in the understory increased with the increase of thinning intensity, and the dominant species in the understory shrub layer changed continuously, while the herbaceous layer remained unchange. (2) The diversity indices of understory plants increased with the thinning intensity, with significant difference between 40% thinning and unthinning, except for Margalef index. (3) The above-ground biomass of shrub layer accounted for the majority of the total biomass and increased with the increase of thinning intensity, while the herbaceous layer showed the opposite trend. (4) The leaf area index decreased with the increase of thinning intensity, and the canopy opening, direct light, scattered light, and total light all increased with thinning, with the significant difference observed at 40% thinning. (5) Soil total nitrogen content increased significantly with the increase of thinning intensity, but soil available nitrogen, phosphorus, potassium, and organic matter contents decreased significantly. (6) Shrub layer diversity index was significantly positively correlated with above-ground biomass and soil total nitrogen of shrub layer, and significantly negatively correlated with above-ground biomass, available phosphorus, and total potassium contents of herbaceous layer. The diversity index of herbaceous layer was positively correlated with canopy opening, direct light, scattered light and total light, while negatively correlated with aboveground biomass and leaf area index of herbaceous layer. In conclusion, thinning reduced the stand density, improved the light environment in the forest, and gradually accumulated the soil TN content, thus promoting the growth of understory plants, increasing the above-ground biomass of understory plants and increasing the understory plant diversity.Above-ground biomass and soil chemistry of understory plants were the main factors affecting the diversity of shrub layer, and stand light environment was the main factor affecting herbaceous layer. In terms of the three thinning intensities used in this study, the optimum thinning intensity of Cunninghamia lanceolata plantation was 40%.