The invention relates to a steam generator and a ring cavity seal device. The ring cavity seal device is arranged in a ring cavity formed between an inner cylinder and an outer cylinder which are sleeved, the ring cavity seal device comprises an inner ring seal unit, and an outer ring seal unit, the inner ring seal unit is arranged along the circumference of the outer wall face of the inner cylinder, the outer ring seal unit is arranged along the circumference of the inner wall face of the outer cylinder, the inner ring seal unit and the outer ring seal unit are in lap joint for seal in the axial direction, at least one of the inner ring seal unit and the outer ring seal unit is provided with a buffer part for supporting the lap-joint position in the axial direction, the buffer part is arranged along the circumference of the ring cavity, and axial compression deforming for buffer is achieved. The inner ring seal unit and the outer ring seal unit of the ring cavity seal device are in lap joint, through elastic supporting of the buffer parts capable of axially deforming, extra bolts do not need to exert seal pre-tightening force, and on through assembling fixing of the inner cylinderand the outer cylinder, and heat expansion force of heated materials, pressing between the inner ring seal unit and the outer ring seal unit is achieved.