This study systematically investigates the influence of integrating KCl into the refining flux on the purification efficiency of Mg–Li alloy. The findings reveal that an optimal purification effect is achieved with a 50 % addition of KCl to the refining flux, resulting in a remarkable 56 % reduction in volume fraction of inclusions to 0.17 % and an 80 % improvement in elongation to 29.1 %. The developed novel quaternary refining flux is composed of KCl + LiCl + LiF + CaF2 (3:3:1:2, wt.%). Based on thermodynamic calculation and analysis, the purification mechanism is elucidated, demonstrating that KCl enhances purification efficiency by reducing flux and melt viscosity, improving wettability, increasing contact area, and facilitating flux adsorption on inclusions. Excessive KCl addition hinders purification by decreasing flux density, slowing settling velocity, and affecting the purification process. The quaternary flux, applying to industrial Mg–Li alloy scrap, demonstrates outstanding purification performance with a 73 % reduction in inclusion volume fraction to 0.33 % and a remarkable 584 % increase in elongation to 17.1 %. The development and application of a novel Mg–Li alloy flux are of significant importance for promoting the industrial development of Mg–Li alloys.