The nano metallic-based material has received the particular attention of scientists in H-2 storage. Herein, an efficient air-stable nano metallic magnesium (Mg)-Polymethyl methacrylate (PMMA) system, in which methyl magnesium chloride (MeMgC1) as organic Mg precursor is in-situ reduced to metallic Mg particles (Mg NPs) by lithium naphthalene (Li-naphthalene) in soluble PMMA/THF system, exhibits an excellent H-2 storage performance and do not require harsh operation condition. In order to form well-distributed Mg NPs (co. 5 nm) in PMMA gel framework, it is an important procedure to mix Mg ion and Li-naphthalene completely, as well the restriction effect of polymer molecular chain. The synthesized mechanism of nanocomposite and the optimal reaction conditions were ascertained by designing a series of experiments. Notably, PMMA can not confined the size of metallic Mg by blending method, and the mixed beta/gamma-Mg presents nearly no ability to adsorb hydrogen. Here, the air stable Mg NPs is in-situ reduced in PMMA can be reacted with H-2, and O-2 and H2O molecules can not be infiltrated into PMMA. The correlation between the size of Mg NPs, the amount of PMMA and hydrogen storage performance for the PMMA-Mg NPs composite (PMC) is studied. We have found that hydrogen storage capacity of PMC could be enhanced as decreasing the size of Mg NPs by adjusting the amount of PMMA. The Mg NPs in PMMA might release the higher amount of H-2 at below 300 degrees C with a rapid absorption/desorption kinetics than the reported material in literature. The obtained nanocomposite are able to deliver dense hydrogen in demanding environments. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.