In the pyrometallurgical treatment for spent lithium-ion batteries (LIBs), lithium is generally present in slag with Al, Ca and Si and is hard to be further treated. In this study, lithium was recovered from a simulated pyrometallurgical slag (pyro-slag) via sodium roasting and water leaching. The thermodynamic process for the reactions between slag and additives such as NaCl, NaNO₃ and Na₂SO₄ were simulated during roasting by the HSC software, where Na₂SO₄ possessed stronger chemical reactivity. The optimal conditions for roasting were experimentally determined to be 800 °C for 60 min and an Na₂SO₄/Li molar ratio of 3 : 1, followed by water leaching at 70 °C for 80 min using a liquid-to-solid (L/S) mass ratio of 30 : 1. This yielded a maximum of 93.62% lithium recovery. The mechanism by which insoluble lithium in slag was transformed into soluble lithium by salt roasting was proposed using the analysis of XRD and EDS spectra, in which ion exchange occurred between Na⁺ and Li⁺ at a certain temperature.