The invention discloses a synchronous excavation supporting system for a deep and large asymmetric pit in a soft soil area and an implementation method of the supporting system. The deep and large asymmetric pit consists of a deep area and a shallow area which are internally close to each other, the top elevation of an enclosure structure at the junction of the deep area and the shallow area is a substrate of the shallow area, the total pit is provided with a first horizontal support in a unified way, the deep area is additionally provided with a plurality of deep-area horizontal supports, a soil body platform used for balancing the horizontal transfer force of the deep-area horizontal supports is reserved at the junction of the interior of the shallow area and the deep area, and two sides of the soil body platform are sloped to the substrate of the shallow area, so that open excavation and bottom-up construction in the deep area and the shallow area can be synchronously carried out, the problem of imbalance of the internal force and the deformation of the pit caused by an asymmetric excavation mode due to function requirements is solved, and the asymmetric pit can be excavated in a safe, reliable, economic and environment-friendly mode; through the synchronous excavation of the deep area and the shallow area, the construction period can be effectively shortened, and the construction cost is reduced; moreover, the supporting system is definitely stressed, the horizontal force of each horizontal support additionally arranged in the deep area is reliably transferred, and the construction feasibility is high.