Cyclic voltammetry and chronoamperometry were used to investigate the electrochemical behavior of Fe(Ⅱ) and Er(Ⅲ) in a LiClO4-DMSO(dimethylsufoxide) system at Pt and Cu electrodes. Experimental results indicate that the reductions of Fe(Ⅱ) to Fe(0) and Er(Ⅲ) to Er(0) were irreversible at Pt and Cu electrodes. The diffusion coefficient and the electron transfer coefficient of Fe(Ⅱ) in a 0.01 mol/L FeCl2-0.1 mol/L LiClO4-DMSO system at 303 K were 1.70×10-10 m2/s and 0.08 respectively, the diffusion coefficient and the electron transfer coefficient of Er(Ⅲ) in a 0.01 mol/L ErCl3-0.1mol/L LiClO4-DMSO system at 303 K were 1.47×10-10 m2/s and 0.108 respectively. The homogeneous, strong adhesive Er-Fe alloy films containing Er of 31.39%-42.12% in mass fraction with metallic lustre were prepared by potentiostatic electrolysis on a Cu electrode in a ErCl3-FeCl2-LiClO4-DMSO system at -1.75--2.50 V(vs. SCE).