In this work, the hydrogen desorption and structural properties of the Li-Mg-N-H systems with different LiH/Mg(NH2)(2) ratios are systemically investigated. The results indicate that the system with the LiH/Mg(NH2)(2) ratio of 6/3 transforms into Li2NH and MgNH, and then, the mixture forms an unknown phase by a solid-solid reaction, which presumably is the ternary imide Li2Mg(NH)(2); the system with the LiH/Mg(NH2)(2) ratio of 8/3 transforms into 4Li(2)NH and Mg3N2 after releasing H-2 at T < 400 degrees C; the system with the LiH/Mg(NH2)(2) ratio of 12/3 transforms into 4Li(3)N and Mg3N2 after releasing H-2 at T > 400 degrees C, where the LiMgN phase is formed by the reaction between Li3N and Mg3N2. The characteristics of the phase transformations and the thermal gas desorption behaviors in these Li-Mg-N-H systems could be reasonably explained by the ammonia mediated reaction model, irrespective of the difference in the LiH/Mg(NH2)(2) ratios.