A kind of assembly kit group, for use as a back cushion, a sitting cushion, and a sitting cushion having a rotatable base, with a sitting cushion cover set to cover the outside section of a cushion body, and this cushion body having a concave hole in the central section, then a combining element combines to the inside of the concave hole in the central section position of the cushion body, so as to allow the central section position of this sitting cushion cover to slide into the inside of this concave hole.