This study revealed the single and mixed decomposition characteristics of fine roots in bamboo forest and its understory vegetation, explored the potential mechanisms of interaction and function between them,and provides a theoretical reference for the rational management of understory vegetation in Moso bamboo forest. The fine-root in situ decomposition and nutrient release processes of Phyllostachys edulis and understory vegetation (Dicranopteris pedata) in Changning, Sichuan, were investigated using a litter bag, and our test period is 1 year.Our results showed that (1) the initial chemical components of the fine roots of P. edulis and D. pedata were significantly different. The C content and C/N and C/P ratios of P. edulis were significantly higher than those of D. pedata (P < 0.05), while the N content, P content, and N/P ratio of D. pedata were all higher than those of P. edulis (P < 0.05). (2) The decomposition coefficients (k) of P. edulis and D. pedata were 0.66±0.04 and 0.42±0.41, respectively. The rate of fine-root decomposition of P. edulis was higher than that of D. pedata. The soil temperature was positively correlated with the decomposition rate, and it was the key environmental factor affecting the decomposition rate of fine roots. (3) The C, N, and P nutrients showed a net release, and the release rate of C in P. edulis was higher than that in D. pedata, while the release rates of N and P in D. pedata were higher than those in P. edulis. (4) The fine-root mixtures of P. edulis and D. pedata had no significant effect on the decomposition rate and release of P, but significantly promoted the release of C and inhibited the release of N at the initial stage of decomposition. The characteristics of single fine-root decomposition and nutrient release were different between P. edulis and D. pedata. No significant mixed effect of decomposition rate was observed, but the mixed effect of nutrient release showed different stages and directions (positive or negative). The understory vegetation affected the nutrient cycle of the bamboo forest ecosystem by affecting the release of nutrients from the fine roots.