Abstract: In this paper, the definitions of open and closed boundary are presented firstly. The differences of both are discussed. Through three samples,the reason for open type boundary being better than closed boundary in reinforced effect is analyzed. Then,it is shown how to convert closed boundary into open boundary. Secondly, it is present that the flexible and rigid sealing membrane can also made up open and closed boundary. The differences of both are discussed. Thirdly, the definitions of sealing wall and the retaining wall, the function and application scope are given. Finally, the cement wall and the geogrid wall forming by cement mixing pile and jet grouting pile are introduced respectively. Through discussing their definition, function and application scope, the former is only as sealing wall, and the latter is not only as sealing wall but also as retaining wall.