Litter decomposition, a key process in material cycling of vegetation ecosystems, is regulated by both litter characteristics and biotic/abiotic environmental factors. Investigation of these mechanisms is crucial for understanding ecosystem functions. This study conducted a 1-year in situ litterbag decomposition experiment in a Robinia pseudoacacia plantation in sub-humid region of Loess Plateau, comparing plots with natural precipitation (control) and 30% precipitation reduction (treated for 4 years), and subplots with and without fine roots exclusion. The results showed that rainfall exclusion significantly decreased decomposition rate of leaf litter, while the effect of fine root exclusion was not significant. After one growing season, mass loss rate of the leaf litter (39.3%) and decomposition coefficient (0.67) in treatment plot with fine roots were significantly lower than those in control plot (43.6% and 0.78, respectively) (P<0.05). The activity of five enzymes in the soil near decomposition bags showed seasonal variation, with higher levels in the early period (May-August) than in the later period (November). However, most sample groups showed lower enzyme activity in treatment plot than in control plot. Sucrase and catalase activities in treatment plot were significantly lower, while polyphenol oxidase activity was higher (P<0.05). The effect of rainfall reduction on alkaline phosphatase activity was not significant. Fine root exclusion significantly decreased sucrase, alkaline phosphatase, and catalase activities, but increased polyphenol oxidase activity (P<0.05). No significant correlation was observed between decomposition rate and soil enzyme activities and major soil nutrient indicators. This study clarified the important role of rainfall reduction and soil moisture conditions in litter decomposition, and supported the potential role of fine roots in litter decomposition. Both soil moisture and fine roots may regulate litter decomposition and nutrient cycling in forests by affecting root exudates and soil microbial activity.