Provided is a lengthening and shortening type steam soot blower. A first sealing mechanism is arranged between a spray-blowing pipe and an air supply pipe. A second sealing mechanism is arranged between the spray-blowing pipe and a furnace wall. The first sealing mechanism comprises a follow-up sealing piece, an auxiliary sealing piece and a sealing filler assembly. An inner sealing ring of the follow-up sealing piece is arranged in an annular sealing groove of an outer ring body and is in clearance fit with the outer wall of the air supply pipe. A flexible sealing ring of the second sealing mechanism is arranged in a sealing cavity of a sealing cavity assembly. An interval slot in the inner side face of the inner sealing ring divides the sealing plane into independent sealing areas. The auxiliary sealing piece is composed of an annular fixing block, a wedge-shaped push plate and a rubber sealing ring. The wedge-shaped push plate is arranged in a wedge-shaped hole in the side face of the annular fixing block. The wedge-shaped face of the wedge-shaped push plate makes contact with the wedge-shaped inner wall of the wedge-shaped hole, and the other side opposite to the wedge-shaped face makes contact with the rubber sealing ring. Abrasion, caused by relative movement, to the sealing pieces is reduced, the problem of leakage caused by squeezing deformation is solved, and the reliability and airtightness of the sealing structure of the steam soot blower are improved.