The invention relates to a dewatering well blocking-off structure and a construction method thereof. The dewatering well blocking-off structure comprises bar-mat reinforcement, the bar-mat reinforcement is horizontally and fixedly assembled on the inner wall of a dewatering well, a sealing material is filled with the lower part of the bar-mat reinforcement, water-stop expansion adhesive tape is laid on the upper surface of the bar-mat reinforcement, the water-stop expansion adhesive tape is fixedly connected with the bar-mat reinforcement, a pouring template is arranged on the upper surface ofthe water-stop expansion adhesive tape, the pouring template is fixedly connected with the side wall of the dewatering well, a grouting material is poured on the top of the pouring template, the topof the grouting material is aligned to the well opening of the dewatering well, a ring-shaped groove is formed in the upper part of the well opening of the dewatering well, a water-stop strip is mounted in the ring-shaped groove, a covering plate is fixed to the upper part of the well opening of the dewatering well, and the covering plate is fixedly connected with concrete of the well opening of the dewatering well through fasteners. The dewatering well blocking-off structure and the construction method have the advantages that economic efficiency, practicality, simpleness and easy-to-practiceare achieved, blocking-off quality is good, bottom plate leakage is prevented, the whole structure is stable, and the carrying capacity is high, the maximum carrying capacity can reach 800 kN, and the dewatering well blocking-off structure and the construction method are mainly used for solving the problem of blocking-off construction of the well opening with large water pressure.