Al-Mg-B films have been the subject of numerous investigations because their attractive properties, including high hardness, high melting point, low density, chemical inertness and thermal stability at high temperature. These materials may be useful in the fields of mechanical, aerospace and military. In this paper, using multi-target magnetron sputtering, we prepared Al-Mg-B amorphous film with atomic ratio approach to Al:Mg:B =1:1:14 on the silicon (100) substrate at room temperature by tuning the sputtering conditions. For the Al-Mg-B films along the AlMg isocontent line, nanoindentation test indicated that the hardness of films increase from 13 GPa to 32 GPa with the contents of B increasing, and the value would reached to 25-32 GPa for AlMgB14. Meanwhile, using first-principles calculations, we estimated the Vickers hardness of AlMgB14 crystal is 27.6 GPa, which agreed to the experimental value. Based on the electron density of states and Mulliken population analysis, the origination of hardness of AlMgB14 crystal other B12-based ternary compounds were discussed. The crystal hardness is primarily determined by the B12 icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms, showing weak covalent bond of Al-B and the ionic characteristics of Mg-B bonds, which is also demonstrated by our experimental XPS core level spectra. Moreover, we also investigated the thermodynamic property of AlMgB14 crystal by Quasi-harmonic Debye model, and found that the bulk modulus decrease with the temperature rising. Such tendency was opposite to the experiment results of Al-Mg-B films have higher hardness under high substrate temperature, which implying the role of temperature is to strengthen the adhesion between film and substrate rather than improving the true hardness of the materials.