Nontoxic Sn-based perovskite light-emitting diodes (Pero-LEDs) have been developing rapidly in recent years. However, high-quality Sn-based perovskite films are hardly prepared because of the heavy self-doping of Sn in the as-prepared films. Most previous reports indicate that the Sn formation is mainly attributed to the Sn oxidation by external oxidizers. Here, for the first time, we reveal that the disproportionation decomposition of Sn during the annealing process plays a critical role in degrading device performance. To resolve this issue, we introduced formamidine thiocyanate into the perovskite precursor as a thermal-sacrificial agent to alleviate the disproportionation decomposition. Finally, we achieved efficient Pero-LEDs with a maximum external quantum efficiency of 5.3% and ultralow efficiency roll-off. This work provides a view of understanding the instability of Sn-based perovskite and presents a practical method to achieve efficient Sn-based Pero-LEDs by suppressing the disproportionation decomposition of Sn2+.