With the increasing interest in transition metal chalcogenides, sulfide minerals containing the disulfide unit (S2 2-) have gained intensive attention for potential applications in energy storage devices, such as lithium-ion batteries (LIBs). Vanadium tetrasulfide (VS4) possesses a unique linear-chain structure with a Peierls distortion and shows great promise for application in LIBs. However, its electrochemical reaction mechanism is still controversial, mainly due to the amorphous nature of the intermediates and final products. Here, by applying multiple X-ray spectroscopies, we reveal that VS4 undergoes lithium intercalation and conversion reactions sequentially during the first discharge process, which are partially reversible in the subsequent charge process. However, an anomalous intercalation/conversion mixed reaction mechanism is dominant for the second cycle, mainly owing to the amorphization of the VS4 electrode during the first cycle. In addition, the sulfur atoms are also involved in the redox reaction during cycling, with the anionic contribution of S2 2- ↔ 2S2- transformation. Furthermore, we find that the formation process of the solid electrolyte interphase is highly dynamic during the discharge and charge processes. The present study provides deeper insights into the complex reaction mechanism of VS4. This knowledge can accelerate the development of high-performance VS4-based electrode materials for LIBs.