Abstract: Exploring the dynamic effects of thinning on the diversity of understory plants in Chinese fir forests can provide a basis for the sustainable management and ecological benefit evaluation of Chinese fir forests. By taking the Chinese fir forest in southwest Zhejiang as the object, the species, importance value and diversity of understory plants after thinning for 1 year, 3 years, 8 years and without thinning (0 a ) were comprehensively investigated and analyzed by using the method of space instead of time. The results showed that there were 37, 48, 59 and 66 species of undergrowth plants after thinning for 0, 1 year, 3 years and 8 years, respectively, of which 35 species were common under Chinese fir forests with different thinning years. With the extension of thinning years, the growth rate of plant species under Chinese fir forests showed a trend of “first fast and then slow”. Compared with 0 a, the species of understory shrubs, herbs and lianas increased by 38.5%−100.0%, 27.8%−72.2% and 16.7%−66.7% after thinning for 1 year, 3 years and 8 years, respectively. The sum of importance values of the top three dominant species of shrubs, herbs and lianas under Chinese fir forests showed a general decreasing tendency with the increase of thinning years. There were no heliophilous plants growing under the unthinned Chinese fir forest, but the species increased to 12～13 after thinning. The species of neutrophilous plants increased with the extension of thinning years. Compared with 0 a, the species of neutrophilous plants increased by 50.0% and 85.0% after thinning for 3 years and 8 years, respectively. The diversity index of understory plants in Chinese fir forest increased with the extension of thinning years. The Simpson dominance index, Shannon-Wiener diversity index and Margalef richness index of shrubs and herbs showed significant differences among different thinning years (P＜0.05). The Simpson, Shannon-Wiener and Margalef of lianas after thinning for 3 and 8 years were significantly higher than those of 0 a (P＜0.05). There was no significant difference in Pielou evenness index of plants among different thinning years (P＞0.05).