Abstract Structural and elastic properties of La2Mg17 with layer structure have been investigated within framework of the density functional theory. Different from the general layer-structured materials, the obtained c/a is less than unity. The calculated elastic constants C33 is larger than C11, being novel in comparison with other alloys with layer structure. The calculated bulk, shear and Young’s modulus of La2Mg17 are higher than other Mg–La alloys with higher La content, implying the stronger covalent bonding. Moreover, the elastic isotropies of La2Mg17 are more excellent. The electronic structure within basal plane is highly symmetric, and the electronic interaction within basal plane is slightly weaker than one between basal planes, which reveal the underlying mechanism for the structural and elastic properties of La2Mg17.