The utility model relates to a pressure-reducing and dewatering supplementary well completion structure in a disturbance stratum inside a foundation pit. A steel pipe protection cylinder is pressed from a trestle into a position of a water-bearing stratum where the stratum is not disturbed because of confined water inrush or other factors, a well pipe is disposed inside the steel pipe protection cylinder, and a steel sleeve is also disposed outside the steel pipe protection cylinder at a place where a large base plate or a cushion layer is disposed. The steel pipe protection cylinder is provided with a slurry returning opening which is slightly higher than the water level of the confined water, a slurry box is disposed on a support platform which is slightly lower than the water level of the confined water, and a slurry pipe of the slurry box is connected with the slurry returning opening. A sediment tube is disposed in a pit bottom of the well pipe, and a filter is disposed above the sediment tube. Three filling layers are disposed between the well pipe and the steel pipe protection cylinder, with a bottom layer being a filtering layer, a middle layer being a water stop layer and a top layer being a concrete layer. By using the structure of the utility models to construct foundation pit excavation projects, even if the situation of stratum disturbance caused by confined water inrush occurs, damages to the large base plate at a periphery of the pit can be prevented and smooth foundation pit excavation and bottom sealing can be ensured.