Woody debris (WD) is an important structural component of many forest ecosystems and plays a key role in carbon sequestration. Despite the large amounts of carbon stored in WD, little attention is paid to its decomposition, which is critical to mitigating climate change, including the chronically elevated atmospheric nitrogen and phosphorus deposition. This study aimed to understand the factors controlling organic matter decomposition in tropical forests. We conducted a factorial fertilization experiment in a tropical forest in South China to investigate the influence of nitrogen (N) and phosphorus (P) availability on WD decomposition using branch segments (5 cm in diameter) of Eucalyptus urophylla in plots fertilized with +N, +P, +NP, and controls. The results showed that nitrogen addition had negative (-11%) effect on E. urophylla WD decomposition, whereas phosphorus addition had positive (+6%) effect. Two-way ANOVA showed that P addition statistically increased WD decomposition rate constants (P = 0.027), indicating that P inputs stimulate WD decomposition and enhance C release. Compared to the control, in the early period of decomposition, N addition promoted N and P release by leaching, while at the middle of decomposition, N addition significantly increased the N and P content remainings (> 100%) but decreased them (< 100%) at the end of decomposition. We observed significant accumulation of P in P addition plots in the early period of decomposition, while at the end of decomposition, all treatments had the similar P content remaining (< 100% for all). Moreover, through the decomposition, WD mass remaining had a significantly negative relationship with both C:N ratio and C:P ratio. Our results indicated that decomposition of WD is primarily constrained by P availability in the tropical forest.