The invention discloses a well sealing device for a high-water-pressure dewatering well and a construction method for the well sealing device. The well sealing device comprises a steel tube, a sleeve, an umbrella sealing mechanism and a pushing-pressing mechanism arranged on the upper part of the steel tube, wherein the sleeve is arranged on the outer part of the steel tube, and a guide slot is formed in the sleeve; the umbrella sealing mechanism comprises an inner bracket, an outer bracket and a circular ring slide block; one end of the inner bracket is hinged with the outer bracket; the other end of the inner bracket is welded on the circular ring slide block; the sleeve and the sleeve are arranged in a concentric mode; a welding block is arranged in the guide slot in the sleeve; the circular ring slide block is fixed with the steel tube by welding through the welding block; a lower flange on the pushing-pressing mechanism is fixedly arranged in a well hole of the dewatering wheel; the lower flange is welded with the sleeve by a blocking ring; a screw is fixedly arranged on the lower flange; an upper flange is fixed with the screw by welding; a dowel pin of the upper flange is connected with the top of the steel tube; and the position of the upper flange can be regulated by a nut on a rotary screw. The well sealing device provided by the invention is reasonable in structure, is convenient to operate, and has the advantage of good sealing property.