Considerable efforts have been devoted to the catalytic modification of hydrogen storage materials. The K-modified Mg(NH2)2/2 LiH composite is a typical model for such studies. In this work, we analyze the origin of the kinetic barrier in the first step of the dehydrogenation and investigate how K catalyzes this heterogeneous solid-state reaction. Our results indicate that the interface reaction of Mg(NH2)2 and LiH is the main source of the kinetic barrier at the early stage of the dehydrogenation for the intensively ball-milled Mg(NH2)2/2 LiH sample. K can effectively activate Mg(NH2)2 as well as promote LiH to participate in the dehydrogenation. Three K species of KH, K2Mg(NH2)4, and Li3K(NH2)4 likely transform circularly in the dehydrogenation (KH↔K2Mg(NH2)4↔KLi3(NH2)4), which creates a more energy-favorable pathway and thus leads to the overall kinetic enhancement. This catalytic role of K in the amide/hydride system is different from the conventional catalysis of transition metals in the alanate system.