Dewatering method in water collecting tank

The invention discloses a dewatering method in a water collecting tank. The method comprises the following steps: pouring a tank bottom cushion layer with a reserved gradient, and reserving an unpoured ground in the middle of the tank bottom cushion layer, wherein the ground is the lowest part of the tank bottom cushion layer; excavating the ground which is not poured to form a pit; arranging an inverted filter layer at the bottom of the pit; arranging a drainage pipe extending out of the pit vertically in the center of the pit, wherein a drainage hole which is horizontally arranged and communicates the interior and the exterior of the drainage pipe is formed in the drainage pipe, and the drainage hole is located in the pit; arranging an inverted filter layer outside the drainage pipe, wherein a water suction pump is connected to the bottom of the drainage pipe through a guide pipe for dewatering; laying geotechnical cloth on the inverted filter layer around the drainage pipe along the periphery of the draina