Abstract: At present, the direct use of the existing electrolytic double spray method or ion thinning method can not prepare the cast aluminum alloy samples with more than 5% (mass fraction) silicon content which are suitable for transmission electron microscope (TEM) observation. The samples prepared by two methods and the combination of the two methods were observed. The results show that with the addition of hydrofluoric acid to the electronic double spary solution could modified methanol nitrate solution, eliminationg the adverse effect of silicon in alloy, the surface quality of samples were improved, but it could not meet the requirements of the thin area. The addition of monoethanolamine to regulate the pH value of the methanol solution of hydrofluoric acid in 6~7 could not improve the surface quality of samples. Aluminum alloy samples were prepared by ion thinning method. When the thickness of the sample was more than 50 mm, the sample became amorphous due to the longer bombardment time. When the thickness of the sample was less than 50 mm, the strength of the sample was low, and it was easy to deform under the action of external force, which was not convenient to observe by TEM for many times. The samples could be observed by TEM, prepared by the combination of electrolyitic double spray method and ion thinning method. This method is a economic and reasonable for preparing the TEM samples of cast aluminum alloy with silicon content more than 5% after non-uniform.