Hi all
  I would like to draw a circle in the 3D space,but i have no idea about.Can anyone help me?,Thanks.
  The picture shows what i want.

Well, you can play with Torus geometry/mesh, or draw one with linesegments.

The mesh approach will give you some “volume” (thickness variation by depth), although the line segment will keep the line’s visual thickness always constant.

Its up to you.

in case you would need to fill it, RegularPolygonGeometry might also be a class to try…

A circle or a ring?

Like rdoi said, a ring would have more “volume”.

Thank you for everyone’s help! ^_^