A backrest assembly includes a rigid frame having an annular body. A connecting plate is interconnected between left and right sides of the annular body above a bottom side of the annular body. A plurality of supporting plates are interconnected between the connecting plate and the bottom side of the annular body. The annular body further includes a ridge extending radially outward from an outer peripheral face thereof and spaced from a front end face of the annular body, forming a ledge between the front end face and the ridge. A flexible board includes a flange formed on a rear side thereof. The flange is mounted around the ledge and abuts against the ridge. The board further includes a plurality of vents extending from a front side thereof through a rear side thereof. Fasteners are extended through engaging holes in the ridge into fixing holes of the board.