A trim cover assembly of extremely soft property comprises a surface cover element, a wadding cover, and a thick foam wadding sandwiched therebetween, wherein the thick foam wadding has a compressed thin end surface region sewn with the surface cover element and wadding cover. Sewing method for forming such trim cover assembly involves: pressing and collapsing an end surface of the thick foam wadding in downwardly slantwise direction to provide a compressed thin area, thereby allowing direct access thereto of a conventional sewing machine; operating the sewing machine to sew the compressed thin area with the surface cover element, thereby producing a sewn trim cover having a compressed thin end region therein; and thereafter sewing the wadding cover with such sewn trim cover in correspondence with the compressed thin end region. As such, a trim cover assembly having the thick foam wadding is easily formed by the conventional sewing machine.