The method of virtual crystal approximation is deployed to control the content of Er in solid solution and the first-principle pseudopotential plane wave (PP W) method based on the density functional theory is uses to study the solid solution strengthening effect of Er on Mg-xEr(x=1 at.%—6 at.%) solid solution. The results indicate that the bulk modulus(B) raises with the increasing content of Er, which peaks at 4at.% Er and remains at almost same level afterwards. Young′s modulus(E) and shear modulus(G) drop with the increment of Er content, while they increase slightly when Er content reaches 6 at.%. The G/B values of six Mg-xEr(x=1 at.% —6 at.%) solid solutions are less than 0.57, all the Mg solid solutions are ductile materials. With the increasing of Er doped amount in the range of 1 at.%—5 at.%, the G/B value of solid solution decreases significantly, while Poisson ratio (ν) increases and the toughness and plasticity of the solid solutions increase. When Er content increases to 6at.%，the G/B value of solid solution decreaseds to some extent, while ν increaseds, and the toughness and plasticity of the solid solutions increase. As the amount of Er increases, the density of state (DOS) moves to the low-energy region as a whole, and the average quantity of bonding electron at low-energy region of Fermi level grows and the bottom band widens simultaneously, resulting in greater bonding capacity. Affected by electrons in the 4f orbital, a clear pseudogap turns up in the total density of state (TDOS) figure when Er content is between 1 at.%—2 at.%. The Fermi level is at the high-energy region of pseudogap where the electronic transition is hard to achieve. When Er content is higher than 2at.%, the pseudogap turns blur and DOS at the Fermi level is high, which means the activity of these solid solutions is increased.