The invention discloses a method for constructing an assembly type pre-stress fish-belly sill internal support system. The method is characterized in that steel bars are planted in the positions, arranged in enclosure walls, of a top plate, and are connected with steel bars in the top plate, when the top plate is cast, lifting holes are reserved in the top plate, after the strength of the top plate is achieved, fish-belly sill supports above the top plate are dismounted firstly, at the moment, the top plate and a bottom plate bear pit outside loads borne by the enclosure walls firstly, then, fish-belly sill supports between the top plate and the bottom plate are dismounted, the dismounted fish-belly sill supports between the top plate and the bottom plate are lifted away through the lifting holes reserved in the top plate, soil coverage is carried out until the ground, and finally side walls are cast. According to the method, the most unfavourable condition that in the order method, before the side walls are cast, the supports between the structural top plate and the structural bottom plate are dismounted, the supports above the structural top plate and structural steel between the structural top plate and the structural bottom plate are oppositely supported, and stress is increased is avoided, and the consumable amount of the fish-belly sill supports and structural steel supports is greatly reduced. Meanwhile, soil coverage is carried out as soon as possible, the bottom heave amount and pit side displacement are reduced, and the method is particularly suitable for foundation pit engineering with the higher requirement.