how can i do this shadow effect

there is a plane which is semitransparency in front of the object, and a light source behind the object, how can I draw the project-shadow on the plane which can be seen
in front of the plane?

You could probably use some of the techniques used in shadowing with shadow volumes. You probably want to find the silhouette edge of the object and then project that onto the plane. Foley, van Dam, et al has a description of this. Gamasutra has a feature from Oct 11 on stencil shadows which might also be helpful.

If you were looking for actual code examples, sorry.