Selective electrodialysis is a promising approach to recovering K+ from complex coexisting ionic systems. In this study, the effects of current density, the concentration of K+ and Mg2+, as well as the operating temperature on the separation process of K+ and Mg2+ were explored to investigate the competitive migration of mono- and multivalent ions, offering a guide for the design of selective electrodialysis process, and therefore obtain the desired aqueous solutions containing K+ and Mg2+. The results show that ion concentration played a critical role in determining the selectivity of separation between K+ and Mg2+. High concentrations of K+ and Mg2+ led to a decrease in selectivity but the effect of concentration of K+ on selectivity was more pronounced. Although higher current density increased the flux of ions, their impact on separation selectivity was minimal. Furthermore, higher temperature increased the flux of ions but resulted in a decrease of K+ proportion in the solution. Overall, this study provides good guidance for studying the competitive migration of mono- and multivalent ions and the high-value recycling of potassium resources.