The lithiation reaction is a key step in anionic graft polymerization of polydienes. However, we observed that polymer degradation occurs concurrently with the lithiation process. Herein, alkyllithium-induced degradation of polydienes was investigated, which showed that the alkyllithium/N,N,N',N'-tetramethylethylenediamine system causes polydiene degradation at room temperature or higher. The number-average molecular weight of rubber decreased from hundreds of thousands to thousands within a few minutes. This reaction occurred in polydienes such as polybutadiene, high-vinyl polybutadiene, polyisoprene, and natural rubber. The effects of the reaction time, temperature, butyllithium dosage, and alkyllithium species in this reaction were studied in detail. Furthermore, the structure of degradation products was characterized by gas chromatography mass spectrometry, gel permeation chromatography, Fourier transform infrared, H-1 nuclear magnetic resonance (NMR), C-13 NMR, two-dimensional heteronuclear single-quantum coherence NMR, and H-2 NMR spectroscopy. Thus, a possible reaction mechanism was proposed, which is described as a process whereby the allylic carbanion undergoes resonance and rearrangement, leading to cleavage of the C-C bond and generation of products with conjugated double bonds at the end group. In addition, the reaction mechanism was also supported by density functional theory calculations, which is revealed as an entropy-driven process.