The effects of Er on the microstructures and the lattice parameters of the as cast Mg-5Sn-xEr (x=0, 0.5, 1, 1.5, 2, 4, 6, 8，10) alloys were investigated by X-ray fluorescence(XRF), optical microscopy(OM), X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS). The addition of Er will result in the grain refinement, inhibite the formation of the phase Mg2Sn, promote the formation of RE-phase with irregular polygon-like shape, so the Er-Adding has obvious effect on Mg-5Sn as cast alloy. The Rietvel whole pattern fitting methods were used to analyze the lattice parameters of α-Mg solid solution in different alloys, on base of it, the Er solubility was calculated with the Vegard’Law on the orientation of a, c axis in α-Mg, respectively, the results show that the calculation of a axis is more reasonable