Abstract: The effective and low-temperature extraction of lithium from the pyrometallurgical slag of spent lithium-ion batteries (LIBs) remains a great challenge. Herein, potassium carbonate/sodium carbonate (K₂CO₃/Na₂CO₃), which could form a eutectic molten salt system at 720°C, was used as a roasting agent to extract lithium from pyrometallurgical slag. Lithium was successfully extracted from the slag by K₂CO₃/Na₂CO₃ roasting followed by water leaching. Theoretical calculation results indicate that the lengths of Li–O bonds increase after K⁺/Na⁺ adsorption, resulting in the easy release of Li⁺ from the LiAlSi₂O₆ lattice after roasting with K₂CO₃/Na₂CO₃. Thermogravimetry–differential scanning calorimetry results indicate that the eutectic phenomenon of K₂CO₃ and Na₂CO₃ could be observed at 720°C and that the reaction of the slag and eutectic molten salts occurs at temperatures above 720°C. X-ray diffraction results suggest that Li⁺ in the slag is exchanged by K⁺ in K₂CO₃ with the concurrent formation of KAlSiO₄, while Na₂CO₃ mainly functions as a fluxing agent. The lithium extraction efficiency can reach 93.87% under the optimal conditions of a roasting temperature of 740°C, roasting time of 30 min, leaching temperature of 50°C, leaching time of 40 min, and water/roasted sample mass ratio of 10:1. This work provides a new system for extracting lithium from the pyrometallurgical slag of spent LIBs.