Little is known about whether soil nutrient supply is determined by decomposition rate or litterfall in Pinus massoniana forests. The present study quantified the relationships between litterfall, litter substrate quality, decomposition rate, and soil nutrients to elucidate the contribution of litter to soil nutrient supply in these forests. Twelve 1 × 1–m litterfall collectors (collected monthly) and 108 decomposition bags, 20 × 20 cm (collected every 3 months) in size, were placed in three 30-year-old P. massoniana stands in the Three Gorges Reservoir Area, from August 2010 to January 2011. The accumulation of soil nutrients depended on the relationship of litterfall, substrate quality, and decomposition rate. Soil organic matter content was significantly positively correlated with litterfall and litter substrate quality dynamic (both P < 0.01) (N, P, and C/N ratio). Litter decomposition rate was positively correlated with litter initial N concentration (R2 = 0.82, P = 0.01) but negatively correlated with initial P (R2 = 0.67, P = 0.045) and C/N ratio (R2 = 0.90, P = 0.00). Litter decomposition rate was negatively correlated with soil organic matter content (P = 0.02), total soil N (P < 0.001), and soil available P (P < 0.001), indicating that litter decomposition was faster in nutrient-poor than in nutrient-rich soils, which may function as a mechanism for increasing the nutrient use efficiency in P. massoniana forest ecosystems. Litter nutrient return through decomposition plays a more important role than the other litter parameters studied in determining soil nutrient supply in the P. massoniana forest ecosystem. Forest management practices need to create conditions that favor the litter decomposition to improve the site conditions and growth rate of the P. massoniana trees.