The invention discloses a foundation pit dewatering system for high-water-level weakly-permeable soil and an implementation method, and belongs to the field of basement dewatering engineering. The system involves a dewatering pipeline, a built-in closure pipe and an externally-arranged motor, wherein a first fixing disc, a second fixing disc, an extrusion plate and an elastic water absorption layer are arranged between the dewatering pipeline and the built-in closure pipe, a water-passing hole is formed in the pipe wall in the middle of the dewatering pipeline, the externally-arranged motor drives a driven wheel connected to the top of the built-in closure pipe to rotate integrally, the extrusion plate extrudes the elastic water absorption layer to realize drainage, and water is absorbed again under the action of pressure difference after drainage is completed, so that the problem of difficult dewatering of the weakly-permeable soil is greatly reduced, and efficient dewatering is achieved; and a sealing ring is arranged between the first fixing disc and the second fixing disc to realize semi-sealing, the elastic water absorption layer generates the large pressure difference which acts on the periphery of the water-passing hole, the water in the soil is collected in the built-in closure pipe and is pumped to the outside of a dewatering well by a small-caliber water pump, a waterlevel control float and a power supply trigger control the working time of the small-caliber water pump, and then the water level in the dewatering well is controlled.