【Objective】This study explored the species diversity of the understory plant community of a Platycladus orientalis plantation across 50 years in Quanshan Mountain and Yunlong Mountain, Xuzhou City. We then investigated the response of the understory plant diversity to the canopy density.【Method】The canopy density of the plantation was measured using a standard sample plot survey method and divided into five canopy density grades ([0.9,1.0], [0.8,0.9),[0.7,0.8),[0.6,0.7),and [0, 0.6) according to the current field stand conditions. The plant species composition and diversity characteristics were then evaluated for different canopy density forests.【Result】The results identified 129 species of trees, shrubs, and grasses in 65 families and 116 genera in the P. orientalis plantation. As the canopy density decreased, the number of plant species initially increased and subsequently decreased, and the understory species exhibited a gradual transition from shade-tolerant to sun-tolerant succession pattern. Plant life forms were generally complex and stable. Moreover, the Shannon-Wiener diversity index initially increased and subsequently decreased with the decrease in canopy closure. The canopy closure values of canopy density categories [0.6, 0.7) and [0.7, 0.8) were significantly higher than those of categories [0.9,1.0],[0.8,0.9) and [0,0.6). No significant differences were observed in the Margalef richness and Pielou evenness indexes among the five canopy densities (P > 0.05). The Simpson dominance index initially increased and subsequently decreased with the decrease in canopy density, and was the highest at the canopy density [0.6, 0.7). Correlation analysis between the canopy density and the plant species diversity index of the P. orientalis plantation revealed a negative correlation between canopy density and the Shannon-Wiener diversity, Margalef richness, and Pielou evenness indexes. Furthermore, the canopy closure was a very weak negative correlated with the Simpson dominance index.【Conclusion】In summary, the plant diversity in the P. orientalis plantation was optimal under medium canopy density [0.6, 0.7). Higher or lower canopy densities may lead to the relative reduction of the species composition and diversity of the plants in the plantation, which is harmful to the maintenance of the P. orientalis plantation.