Abstract: Interplanting the traditional Chinese medicinal herbs such as Polygonatum cyrtonema and Bletilla striata under forest is one of the main modes of under-forest economy, which has good social and economic benefits, but the impact of interplanting on the understory plant diversity is still unclear. By taking the Cunninghamia lanceolata forest interplanted with Polygonatum cyrtonema and Bletilla striata for four years in southwest Zhejiang and the original ecological forest land without interplanting (CK) as the objects, the composition, species and quantity of the understory shrubs, herbs and vines were studied by using the standard land method. The results showed that after 4 years of interplanting Polygonatum cyrtonema and Bletilla striata, the species of herbaceous plants under Cunninghamia lanceolata forest increased significantly by 17.4%-26.1% (P<0.05), while the species of vine plants decreased significantly by 33.3% (P<0.05). The changes of species dominance index Simpson, diversity index Shannon-Weiner and evenness index Pielou significantly increased by 26.4%-36.1% for the herbaceous plants (P<0.05), while decreased by 29.0%-39.3% for the vine plants (P<0.05). The response of understory shrub species and diversity index to interplanting was not obvious. After interplanting, the species of woody shrubs, vines and plants with higher plant types decreased, while the species of plants with smaller plant types increased. In the future, it was necessary to establish the long-term location monitoring to grasp the long-term effects of interplanting Chinese medicinal herbs on the diversity of understory plants, so as to provide a basis for the benefit evaluation of understory planting.