High-energy density and low cost magnesium nanoparticles (Mg NPs)-based material are being sought to meet increasing capable of hydrogen (H-2) storage demand. Here, a kind of air-stable Mg NPs supported on porous structured multi-walled carbon tubes-polymethyl methacrylate (MWCNTs-PMMA) template is prepared owing to reversible well distributed, dispersed and small-sized Mg/MgH2 NPs. The aim is to improve the H-2 storage capacity, hydrogen sorption kinetics and thermodynamics of nano Mg -based system without using catalyst. The organic Mg precursor was directly in-situ reduced to metallic Mg NPs in MWCNTs-PMMA template by lithium naphthalide. The size distribution of reduced Mg nanoparticles is around 3.6 +/- 0.2 nm, confirmed by XRD and TEM analyses, which is due to the strong interaction between Mg NPs and MWCNTs-PMMA via PMMA binding Mg2+, as well as the confinement of porous template hindered the growth and agglomeration of Mg NPs. Moreover, except H-2, O-2 and H2O molecules can't infiltrate the porous structure of MWCNTs-PMMA resulted in the presence of air stable Mg NPs in the MWCNTs-PMMA. The work provides a new scope to prepare nano metal-based composite for H-2 storage. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.