The invention relates to an underwater basal disc connecting a floating production system and a subsea pipeline. The underwater basal disc comprises a basal disc body. The underwater basal disc is improved in the following way: the basal disc body comprises a bottom basic structure and an upper main structure, wherein the bottom basic structure is a gravity type anti-settling plate which consists of an anti-settling plate body and a plurality of skirtboards arranged at the bottom of the anti-settling plate body. The anti-settling plate body is uniformly provided with a plurality of drainage holes penetrating through the plate body. The upper main structure consists of main beams and auxiliary beams, wherein the main beams are a plurality of joist steels which are distributed and fixed on the top face of the anti-settling plate body in a cross manner, and the auxiliary beams are a plurality of joist steels which are distributed and fixed on the top face of the anti-settling plate body and respectively penetrate through a web of the main beam. Process pipelines and subsea cable brackets are arranged on the man beams. Basal disc body hoisting points are arranged at the cross points of the main beams. By the underwater basal disc, the subsea pipeline is transited form a hard sea pipe to a dynamic flexible vertical tube, so that the dynamic sea cable is transited to a stationary sea cable. The skirtboards are pressed into sea mud under dead-weight effect, so that the stability of connection and transition of the flexible vertical tube and the dynamic sea cable system at the seabed is guaranteed.