Abstract: Fallen flowers are an important component of forest litter and clarifying their role in the mixed decomposition of forest litter is important for understanding and forecasting the nutrient cycle in forests. In the present study, fallen flowers and leaf litter produced from a 33-year Robinia pseudoacacia L. plantation were collected. Pure leaf litter and mixed litter (leaves with flowers at proportions of 30%, 20%, 10%, and 5%) were incubated for 62 d to conduct an early-stage decomposition test using soil microbial infection under controlled conditions (20℃–25℃, photophobic, constant humidity). The decomposition rate and carbon (C), nitrogen (N), and phosphorus (P) release rates of the leaf litter and flower mixtures were investigated. Results indicated that: (1) When the proportion of flowers reached 10%, the decomposition rate of the mixed litter was significantly higher than that of the pure leaf litter, and when the proportion reached 20%–30%, the decomposition rate was significantly higher again. However, the mixing of leaves and flowers did not have a significant non-additive effect on decomposition. (2) When the proportion of flowers in the mixture was 10%–20%, the C and N release rates were significantly higher than that of pure leaf litter. In addition, the C and N release rates tended to increase with an increasing proportion of flowers. During the early stages of decomposition, the P release rates tended to be significantly higher for the mixtures with higher flower proportions than those with lower flower proportions or pure leaf litter but exhibited the opposite trend with increasing decomposition. The mixed decomposition of leaf litter and flowers tended to promote C and N release synergistically, showing an increased tendency with increasing proportion of flowers. However, mixed decomposition had an antagonistic inhibitory effect on P release from litter, showing a weakened then enhanced trend with increasing proportion of flowers.