Abstract: In 2019, Polygonatum cyrtonema and Paris polyphylla were interplanted under mature Cunninghamia lanceolata plantations in Daji Forest Farm and Caoyutang Forest Farm in Jingning She Autonomous county, Zhejiang province. In 2021, ten sample plots with underplanting and ten ones without undersplanting were established, and one analytic wood was selected in each sample plot. Based on the consecutive annual diameter growth over the past three years and fitness tests, linear models were established to analyze the impact of underplanting on DBH growth of the plantation. The results indicated that replacement of plantation age increased the quality of the linear model after eliminating non-linearity. The observation showed that underplanting in the forest had a significant positive effect on DBH growth.