The invention discloses a construction method of a dewatering well under a foundation slab in a foundation pit, comprising the following steps of: arranging the dewatering well in the foundation pit, wherein the dewatering well is not damaged in the process of excavating a foundation pit earthwork, and the effectiveness of the dewatering well is ensured; after the foundation pit earthwork is excavated to a basement, excavating interlaced ditches along the center line of the dewatering well, and paving water-filtering type geotextile in the ditches; then paving drain pipes of the dewatering well and cables in the ditches and connecting to the outside of the pit, checking the depth of the dewatering well, an extraction pump and a pipeline thereof to ensure the integrality and the reliability; then paving granular crushed stones or cobbles in the ditches, covering asphalt felt thereon to form a blind ditch; leading the blind ditch to a water collection pit at the edge of the foundation pit and arranging a surface drain on the periphery of the foundation pit; paving a subcrust and a waterproof layer under the constructed foundation slab, gradually reducing the usage quantity and the dewatering speed of dewatering wells in the foundation pit in the construction process of underground structures and upper structures thereof, and assisting in draining by adopting the water collectionpit at the periphery of the foundation pit until the dewatering operation in the foundation pit can be stopped. The invention has convenient construction, safety and reliability and low cost for dewatering in the foundation pit.