how can i select vertex of one side without selecting the other side' vertex? it seems blender always selects both two sides' vertex.

Ask these questions at www.elysiun.com for better response.

Go into a view (numpad 1,3,7 or ctrl-1,3,7) where you can see te back verts, box select them and hide them with 'H'. Or select only the verts you want to work with, hit 'Select Swap' in edit buttons or specials menu and hide them.

%<