The invention discloses a sinking construction method for a prefabricated post-cast open caisson. The method comprises the following steps that an anchor pile is embedded in a soil body below an opencaisson designed elevation; a plurality of prefabricated well wall blocks are assembled into the open caisson on the construction site, a plurality of vertical through holes penetrating through the well wall are circumferentially formed in the well wall of the open caisson at intervals, anchor ropes are arranged in all the through holes in a penetrating mode, the lower ends of the anchor ropes areconnected with the anchor piles, the upper ends of the anchor ropes penetrates through a center hole jack located in the upper end face of the open caisson and are fixed through anchor nuts; and thecenter hole jack applies downward pressure to the open caisson so as to sink the open caisson into the soil body. The method has the advantages that through vertical sectioning and transverse blocking, smallest assembly units, namely the well wall blocks can be prefabricated in a factory, and the construction period of the open caisson is shortened; a cast site required by site open caisson assembling on the open caisson construction site is reduced; and the anchor piles, the anchor ropes and the center hole jack form an open caisson construction device for providing sinking force for the opencaisson, and the sinking gesture of the open caisson can be effectively controlled.