how to convert polygon of postgresql to autocad

Since you mention postgresql I'm assuming it's not stored as a geometry column but rather as "a column with a geometry description". In that case it depends on how it's stored, if it's one of the flavors the GeometryReplacer understands you can that to build the geometry and write out to AutoCAD.

It could also be that you have the PostGIS extension, in which case you can use the PostGIS reader in FME, that will understand the geometry and then writing out to AutoCAD should be fairly easy. Depending on what your requirements are the Quick Translator might even be sufficient.

I would start by using the generate workspace option in workbench.